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Condom

A condom is a sheath-shaped barrier device used during sexual intercourse to reduce the probability of pregnancy or a sexually transmitted infection (STI). There are both external (male) and internal (female) condoms.

The external (male) condom is rolled onto an erect penis before intercourse and works by forming a physical barrier which blocks semen from entering the body of a sexual partner. External condoms are typically made from latex and, less commonly, from polyurethane, polyisoprene, or lamb intestine. External condoms have the advantages of ease of use, ease of access, and few side effects. Individuals with latex allergy should use condoms made from a material other than latex, such as polyurethane. Internal condoms are typically made from polyurethane and may be used multiple times.

With proper use—and use at every act of intercourse—women whose partners use external condoms experience a 2% per-year pregnancy rate. With typical use, the rate of pregnancy is 18% per-year. Their use greatly decreases the risk of gonorrhea, chlamydia, trichomoniasis, hepatitis B, and HIV/AIDS. To a lesser extent, they also protect against genital herpes, human papillomavirus (HPV), and syphilis.

Condoms as a method of preventing STIs have been used since at least 1564. Rubber condoms became available in 1855, followed by latex condoms in the 1920s. It is on the World Health Organization's List of Essential Medicines. As of 2019, globally around 21% of those using birth control use the condom, making it the second-most common method after female sterilization (24%). Rates of condom use are highest in East and Southeast Asia, Europe and North America. About six to nine billion are sold a year.

Birth control
The effectiveness of condoms, as of most forms of contraception, can be assessed two ways. Perfect use or method effectiveness rates only include people who use condoms properly and consistently. Actual use, or typical use effectiveness rates are of all condom users, including those who use condoms incorrectly or do not use condoms at every act of intercourse. Rates are generally presented for the first year of use. Most commonly the Pearl Index is used to calculate effectiveness rates, but some studies use decrement tables.

The typical use pregnancy rate among condom users varies depending on the population being studied, ranging from 10 to 18% per year. The perfect use pregnancy rate of condoms is 2% per year. Condoms may be combined with other forms of contraception (such as spermicide) for greater protection.

Sexually transmitted infections
Condoms are widely recommended for the prevention of sexually transmitted infections (STIs). They have been shown to be effective in reducing infection rates in both men and women. While not perfect, the condom is effective at reducing the transmission of organisms that cause AIDS, genital herpes, cervical cancer, genital warts, syphilis, chlamydia, gonorrhea, and other diseases. Condoms are often recommended as an adjunct to more effective birth control methods (such as IUD) in situations where STI protection is also desired. For this reason, condoms are frequently used by those in the swinging community.

According to a 2000 report by the National Institutes of Health (NIH), consistent use of latex condoms reduces the risk of HIV transmission by approximately 85% relative to risk when unprotected, putting the seroconversion rate (infection rate) at 0.9 per 100 person-years with condom, down from 6.7 per 100 person-years. Analysis published in 2007 from the University of Texas Medical Branch and the World Health Organization found similar risk reductions of 80–95%.

The 2000 NIH review concluded that condom use significantly reduces the risk of gonorrhea for men. A 2006 study reports that proper condom use decreases the risk of transmission of human papillomavirus (HPV) to women by approximately 70%. Another study in the same year found consistent condom use was effective at reducing transmission of herpes simplex virus-2, also known as genital herpes, in both men and women.

Although a condom is effective in limiting exposure, some disease transmission may occur even with a condom. Infectious areas of the genitals, especially when symptoms are present, may not be covered by a condom, and as a result, some diseases like HPV and herpes may be transmitted by direct contact. The primary effectiveness issue with using condoms to prevent STIs, however, is inconsistent use.

Condoms may also be useful in treating potentially precancerous cervical changes. Exposure to human papillomavirus, even in individuals already infected with the virus, appears to increase the risk of precancerous changes. The use of condoms helps promote regression of these changes. In addition, researchers in the UK suggest that a hormone in semen can aggravate existing cervical cancer, condom use during sex can prevent exposure to the hormone.

Causes of failure
Condoms may slip off the penis after ejaculation, break due to improper application or physical damage (such as tears caused when opening the package), or break or slip due to latex degradation (typically from usage past the expiration date, improper storage, or exposure to oils). The rate of breakage is between 0.4% and 2.3%, while the rate of slippage is between 0.6% and 1.3%. Even if no breakage or slippage is observed, 1–3% of women will test positive for semen residue after intercourse with a condom. Failure rates are higher for anal sex, and until 2022, condoms were only approved by the FDA for vaginal sex. The One Male Condom received FDA approval for anal sex on 23 February 2022.

"Double bagging", using two condoms at once, is often believed to cause a higher rate of failure due to the friction of rubber on rubber. This claim is not supported by research. The limited studies that have been done found that the simultaneous use of multiple condoms decreases the risk of condom breakage.

Different modes of condom failure result in different levels of semen exposure. If a failure occurs during application, the damaged condom may be disposed of and a new condom applied before intercourse begins – such failures generally pose no risk to the user. One study found that semen exposure from a broken condom was about half that of unprotected intercourse; semen exposure from a slipped condom was about one-fifth that of unprotected intercourse.

Standard condoms will fit almost any penis, with varying degrees of comfort or risk of slippage. Many condom manufacturers offer "snug" or "magnum" sizes. Some manufacturers also offer custom sized-to-fit condoms, with claims that they are more reliable and offer improved sensation/comfort. Some studies have associated larger penises and smaller condoms with increased breakage and decreased slippage rates (and vice versa), but other studies have been inconclusive.

It is recommended for condoms manufacturers to avoid very thick or very thin condoms, because they are both considered less effective. Some authors encourage users to choose thinner condoms "for greater durability, sensation, and comfort", but others warn that "the thinner the condom, the smaller the force required to break it".

Experienced condom users are significantly less likely to have a condom slip or break compared to first-time users, although users who experience one slippage or breakage are more likely to suffer a second such failure. An article in Population Reports suggests that education on condom use reduces behaviors that increase the risk of breakage and slippage. A Family Health International publication also offers the view that education can reduce the risk of breakage and slippage, but emphasizes that more research needs to be done to determine all of the causes of breakage and slippage.

Among people who intend condoms to be their form of birth control, pregnancy may occur when the user has sex without a condom. The person may have run out of condoms, or be traveling and not have a condom with them, or dislike the feel of condoms and decide to "take a chance". This behavior is the primary cause of typical use failure (as opposed to method or perfect use failure).

Another possible cause of condom failure is sabotage. One motive is to have a child against a partner's wishes or consent. Some commercial sex workers from Nigeria reported clients sabotaging condoms in retaliation for being coerced into condom use. Using a fine needle to make several pinholes at the tip of the condom is believed to significantly impact on their effectiveness. Cases of such condom sabotage have occurred.

Side effects
The use of latex condoms by people with an allergy to latex can cause allergic symptoms, such as skin irritation. In people with severe latex allergies, using a latex condom can potentially be life-threatening. Repeated use of latex condoms can also cause the development of a latex allergy in some people. Irritation may also occur due to spermicides that may be present.

Use
External condoms are usually packaged inside a foil or plastic wrapper, in a rolled-up form, and are designed to be applied to the tip of the penis and then unrolled over the erect penis. It is important that the closed end or the teat of the condom is pinched when the condom is placed on the tip of the penis. This will ensure that air is not trapped inside the condom which could cause it to burst during intercourse. In addition, this leaves space for the semen to collect which reduces the risk of it being forced out of the base of the device. Most condoms have a teat end for this purpose. Soon after ejaculating, the male should withdraw from his partner's body. The condom should then be carefully removed from the penis away from the other partner. It is recommended that the condom be wrapped in tissue or tied in a knot, then disposed of in a trash receptacle. Condoms are used to reduce the likelihood of pregnancy during intercourse and to reduce the likelihood of contracting sexually transmitted infections (STIs). Condoms are also used during fellatio to reduce the likelihood of contracting STIs.

Some couples find that putting on a condom interrupts sex, although others incorporate condom application as part of their foreplay. Some men and women find the physical barrier of a condom dulls sensation. Advantages of dulled sensation can include prolonged erection and delayed ejaculation; disadvantages might include a loss of some sexual excitement. Advocates of condom use also cite their advantages of being inexpensive, easy to use, and having few side effects.

Adult film industry
In 2012 proponents gathered 372,000 voter signatures through a citizens' initiative in Los Angeles County to put Measure B on the 2012 ballot. As a result, Measure B, a law requiring the use of condoms in the production of pornographic films, was passed. This requirement has received much criticism and is said by some to be counter-productive, merely forcing companies that make pornographic films to relocate to other places without this requirement. Producers claim that condom use depresses sales.

Sex education
Condoms are often used in sex education programs, because they have the capability to reduce the chances of pregnancy and the spread of some sexually transmitted infections when used correctly. A recent American Psychological Association (APA) press release supported the inclusion of information about condoms in sex education, saying "comprehensive sexuality education programs ... discuss the appropriate use of condoms", and "promote condom use for those who are sexually active."

In the United States, teaching about condoms in public schools is opposed by some religious organizations. Planned Parenthood, which advocates family planning and sex education, argues that no studies have shown abstinence-only programs to result in delayed intercourse, and cites surveys showing that 76% of American parents want their children to receive comprehensive sexuality education including condom use.

Infertility treatment
Common procedures in infertility treatment such as semen analysis and intrauterine insemination (IUI) require collection of semen samples. These are most commonly obtained through masturbation, but an alternative to masturbation is use of a special collection condom to collect semen during sexual intercourse.

Collection condoms are made from silicone or polyurethane, as latex is somewhat harmful to sperm. Some religions prohibit masturbation entirely. Also, compared with samples obtained from masturbation, semen samples from collection condoms have higher total sperm counts, sperm motility, and percentage of sperm with normal morphology. For this reason, they are believed to give more accurate results when used for semen analysis, and to improve the chances of pregnancy when used in procedures such as intracervical or intrauterine insemination. Adherents of religions that prohibit contraception, such as Catholicism, may use collection condoms with holes pricked in them.

For fertility treatments, a collection condom may be used to collect semen during sexual intercourse where the semen is provided by the woman's partner. Private sperm donors may also use a collection condom to obtain samples through masturbation or by sexual intercourse with a partner and will transfer the ejaculate from the collection condom to a specially designed container. The sperm is transported in such containers, in the case of a donor, to a recipient woman to be used for insemination, and in the case of a woman's partner, to a fertility clinic for processing and use. However, transportation may reduce the fecundity of the sperm. Collection condoms may also be used where semen is produced at a sperm bank or fertility clinic.

Condom therapy is sometimes prescribed to infertile couples when the female has high levels of antisperm antibodies. The theory is that preventing exposure to her partner's semen will lower her level of antisperm antibodies, and thus increase her chances of pregnancy when condom therapy is discontinued. However, condom therapy has not been shown to increase subsequent pregnancy rates.

Other uses
Condoms excel as multipurpose containers and barriers because they are waterproof, elastic, durable, and (for military and espionage uses) will not arouse suspicion if found.

Ongoing military utilization began during World War II, and includes covering the muzzles of rifle barrels to prevent fouling, the waterproofing of firing assemblies in underwater demolitions, and storage of corrosive materials and garrotes by paramilitary agencies.

Condoms have also been used to smuggle alcohol, cocaine, heroin, and other drugs across borders and into prisons by filling the condom with drugs, tying it in a knot and then either swallowing it or inserting it into the rectum. These methods are very dangerous and potentially lethal; if the condom breaks, the drugs inside become absorbed into the bloodstream and can cause an overdose.

Medically, condoms can be used to cover endovaginal ultrasound probes, or in field chest needle decompressions they can be used to make a one-way valve.

Condoms have also been used to protect scientific samples from the environment, and to waterproof microphones for underwater recording.

Types
Most condoms have a reservoir tip or teat end, making it easier to accommodate the man's ejaculate. Condoms come in different sizes and shapes.

They also come in a variety of surfaces intended to stimulate the user's partner. Condoms are usually supplied with a lubricant coating to facilitate penetration, while flavored condoms are principally used for oral sex. As mentioned above, most condoms are made of latex, but polyurethane and lambskin condoms also exist.

Internal condom

External condoms have a tight ring to form a seal around the penis, while internal condoms usually have a large stiff ring to prevent them from slipping into the body orifice. The Female Health Company produced an internal condom that was initially made of polyurethane, but newer versions are made of nitrile rubber. Medtech Products produces an internal condom made of latex.

Natural latex

Latex has outstanding elastic properties: Its tensile strength exceeds 30 MPa, and latex condoms may be stretched in excess of 800% before breaking. In 1990 the ISO set standards for condom production (ISO 4074, Natural latex rubber condoms), and the EU followed suit with its CEN standard (Directive 93/42/EEC concerning medical devices). Every latex condom is tested for holes with an electric current. If the condom passes, it is rolled and packaged. In addition, a portion of each batch of condoms is subject to water leak and air burst testing.

While the advantages of latex have made it the most popular condom material, it does have some drawbacks. Latex condoms are damaged when used with oil-based substances as lubricants, such as petroleum jelly, cooking oil, baby oil, mineral oil, skin lotions, suntan lotions, cold creams, butter or margarine. Contact with oil makes latex condoms more likely to break or slip off due to loss of elasticity caused by the oils. Additionally, latex allergy precludes use of latex condoms and is one of the principal reasons for the use of other materials. In May 2009, the U.S. Food and Drug Administration (FDA) granted approval for the production of condoms composed of Vytex, latex that has been treated to remove 90% of the proteins responsible for allergic reactions. An allergen-free condom made of synthetic latex (polyisoprene) is also available.

Synthetic
The most common non-latex condoms are made from polyurethane. Condoms may also be made from other synthetic materials, such as AT-10 resin, and most polyisoprene.

Polyurethane condoms tend to be the same width and thickness as latex condoms, with most polyurethane condoms between 0.04 mm and 0.07 mm thick.

Polyurethane can be considered better than latex in several ways: it conducts heat better than latex, is not as sensitive to temperature and ultraviolet light (and so has less rigid storage requirements and a longer shelf life), can be used with oil-based lubricants, is less allergenic than latex, and does not have an odor. Polyurethane condoms have gained FDA approval for sale in the United States as an effective method of contraception and HIV prevention, and under laboratory conditions have been shown to be just as effective as latex for these purposes.

However, polyurethane condoms are less elastic than latex ones, and may be more likely to slip or break than latex, lose their shape or bunch up more than latex, and are more expensive.

Polyisoprene is a synthetic version of natural rubber latex. While significantly more expensive, it has the advantages of latex (such as being softer and more elastic than polyurethane condoms) without the protein which is responsible for latex allergies. Unlike polyurethane condoms, they cannot be used with an oil-based lubricant.

Lambskin
Condoms made from sheep intestines, labeled "lambskin", are also available. Although they are generally effective as a contraceptive by blocking sperm, studies have found that they are less effective than latex in preventing the transmission of sexually transmitted infections because of pores in the material. This is because intestines, by their nature, are porous, permeable membranes, and while sperm are too large to pass through the pores, viruses—such as HIV, herpes, and genital warts—are small enough to pass.

As a result of laboratory data on condom porosity, in 1989, the FDA began requiring lambskin condom manufacturers to indicate that the products were not to be used for the prevention of sexually transmitted infections. The FDA cautions that while lambskin condoms "provide good birth control and a varying degree of protection against some, but not all, sexually transmitted diseases", people do not know what STIs a partner might have, and thus cannot assume that a lambskin condom will protect them.

While lambskin condoms avoid triggering latex allergies, polyurethane condoms do as well, while also protecting more reliably against STIs. Lambskin condoms' can be as much as quadruple latex condoms'. As slaughter by-products, lambskin condoms are also not vegetarian. Pharmacist advice prepared by the Canadian Pharmaceutical Journal says that lambskin condoms "are generally not recommended" due to limited STI prevention. An article in Adolescent Medicine advises that they "should be used only for pregnancy prevention".

Spermicide
Some latex condoms are lubricated at the manufacturer with a small amount of a nonoxynol-9, a spermicidal chemical. According to Consumer Reports, condoms lubricated with spermicide have no additional benefit in preventing pregnancy, have a shorter shelf life, and may cause urinary tract infections in women. In contrast, application of separately packaged spermicide is believed to increase the contraceptive efficacy of condoms.

Nonoxynol-9 was once believed to offer additional protection against STIs (including HIV) but recent studies have shown that, with frequent use, nonoxynol-9 may increase the risk of HIV transmission. The World Health Organization says that spermicidally lubricated condoms should no longer be promoted. However, it recommends using a nonoxynol-9 lubricated condom over no condom at all. , nine condom manufacturers have stopped manufacturing condoms with nonoxynol-9 and Planned Parenthood has discontinued the distribution of condoms so lubricated.

Ribbed and studded
Textured condoms include studded and ribbed condoms which can provide extra sensations to both partners. The studs or ribs can be located on the inside, outside, or both; alternatively, they are located in specific sections to provide directed stimulation to either the G-spot or frenulum. Many textured condoms which advertise "mutual pleasure" also are bulb-shaped at the top, to provide extra stimulation to the penis. Some women experience irritation during vaginal intercourse with studded condoms.

Other
The anti-rape condom is another variation designed to be worn by women. It is designed to cause pain to the attacker, hopefully allowing the victim a chance to escape.

A collection condom is used to collect semen for fertility treatments or sperm analysis. These condoms are designed to maximize sperm life.

Some condom-like devices are intended for entertainment only, such as glow-in-the dark condoms. These novelty condoms may not provide protection against pregnancy and STIs.

In February 2022, the U.S. Food and Drug Administration (FDA) approved the first condoms specifically indicated to help reduce transmission of sexually transmitted infections (STIs) during anal intercourse.

Prevalence
The prevalence of condom use varies greatly between countries. Most surveys of contraceptive use are among married women, or women in informal unions. Japan has the highest rate of condom usage in the world: in that country, condoms account for almost 80% of contraceptive use by married women. On average, in developed countries, condoms are the most popular method of birth control: 28% of married contraceptive users rely on condoms. In the average less-developed country, condoms are less common: only 6–8% of married contraceptive users choose condoms.

Before the 19th century
Whether condoms were used in ancient civilizations is debated by archaeologists and historians. In ancient Egypt, Greece, and Rome, pregnancy prevention was generally seen as a woman's responsibility, and the only well documented contraception methods were female-controlled devices. In Asia before the 15th century, some use of glans condoms (devices covering only the head of the penis) is recorded. Condoms seem to have been used for contraception, and to have been known only by members of the upper classes. In China, glans condoms may have been made of oiled silk paper, or of lamb intestines. In Japan, condoms called Kabuto-gata (甲形) were made of tortoise shell or animal horn. In 16th-century Italy, anatomist and physician Gabriele Falloppio wrote a treatise on syphilis. The earliest documented strain of syphilis, first appearing in Europe in a 1490s outbreak, caused severe symptoms and often death within a few months of contracting the disease. Falloppio's treatise is the earliest uncontested description of condom use: it describes linen sheaths soaked in a chemical solution and allowed to dry before use. The cloths he described were sized to cover the glans of the penis, and were held on with a ribbon. Falloppio claimed that an experimental trial of the linen sheath demonstrated protection against syphilis.

After this, the use of penis coverings to protect from disease is described in a wide variety of literature throughout Europe. The first indication that these devices were used for birth control, rather than disease prevention, is the 1605 theological publication De iustitia et iure (On justice and law) by Catholic theologian Leonardus Lessius, who condemned them as immoral. In 1666, the English Birth Rate Commission attributed a recent downward fertility rate to use of "condons", the first documented use of that word or any similar spelling. Other early spellings include "condam" and "quondam", from which the Italian derivation guantone has been suggested, from guanto, "a glove".

In addition to linen, condoms during the Renaissance were made out of intestines and bladder. In the late 16th century, Dutch traders introduced condoms made from "fine leather" to Japan. Unlike the horn condoms used previously, these leather condoms covered the entire penis.

Casanova in the 18th century was one of the first reported using "assurance caps" to prevent impregnating his mistresses.

From at least the 18th century, condom use was opposed in some legal, religious, and medical circles for essentially the same reasons that are given today: condoms reduce the likelihood of pregnancy, which some thought immoral or undesirable for the nation; they do not provide full protection against sexually transmitted infections, while belief in their protective powers was thought to encourage sexual promiscuity; and, they are not used consistently due to inconvenience, expense, or loss of sensation.

Despite some opposition, the condom market grew rapidly. In the 18th century, condoms were available in a variety of qualities and sizes, made from either linen treated with chemicals, or "skin" (bladder or intestine softened by treatment with sulfur and lye). They were sold at pubs, barbershops, chemist shops, open-air markets, and at the theater throughout Europe and Russia. They later spread to America, although in every place there were generally used only by the middle and upper classes, due to both expense and lack of sex education.

1800 through 1920s
The early 19th century saw contraceptives promoted to the poorer classes for the first time. Writers on contraception tended to prefer other birth control methods to the condom. By the late 19th century, many feminists expressed distrust of the condom as a contraceptive, as its use was controlled and decided upon by men alone. They advocated instead for methods controlled by women, such as diaphragms and spermicidal douches. Other writers cited both the expense of condoms and their unreliability (they were often riddled with holes and often fell off or tore). Still, they discussed condoms as a good option for some and the only contraceptive that protects from disease.

Many countries passed laws impeding the manufacture and promotion of contraceptives. In spite of these restrictions, condoms were promoted by traveling lecturers and in newspaper advertisements, using euphemisms in places where such ads were illegal. Instructions on how to make condoms at home were distributed in the United States and Europe. Despite social and legal opposition, at the end of the 19th century the condom was the Western world's most popular birth control method.

Beginning in the second half of the 19th century, American rates of sexually transmitted infections skyrocketed. Causes cited by historians include the effects of the American Civil War and the ignorance of prevention methods promoted by the Comstock laws. To fight the growing epidemic, sex education classes were introduced to public schools for the first time, teaching about venereal diseases and how they were transmitted. They generally taught abstinence was the only way to avoid sexually transmitted infections. Condoms were not promoted for disease prevention because the medical community and moral watchdogs considered STIs to be punishment for sexual misbehavior. The stigma against people with these diseases was so significant that many hospitals refused to treat people with syphilis.

The German military was the first to promote condom use among its soldiers in the later 19th century. Early 20th century experiments by the American military concluded that providing condoms to soldiers significantly lowered rates of sexually transmitted infections. During World War I, the United States and (at the beginning of the war only) Britain were the only countries with soldiers in Europe who did not provide condoms and promote their use.

In the decades after World War I, there remained social and legal obstacles to condom use throughout the U.S. and Europe. Founder of psychoanalysis Sigmund Freud opposed all methods of birth control because their failure rates were too high. Freud was especially opposed to the condom because he thought it cut down on sexual pleasure. Some feminists continued to oppose male-controlled contraceptives such as condoms. In 1920 the Church of England's Lambeth Conference condemned all "unnatural means of conception avoidance". The Bishop of London, Arthur Winnington-Ingram, complained of the huge number of condoms discarded in alleyways and parks, especially after weekends and holidays.

However, European militaries continued to provide condoms to their members for disease protection, even in countries where they were illegal for the general population. Through the 1920s, catchy names and slick packaging became an increasingly important marketing technique for many consumer items, including condoms and cigarettes. Quality testing became more common, involving filling each condom with air followed by one of several methods intended to detect loss of pressure. Worldwide, condom sales doubled in the 1920s.

Rubber and manufacturing advances
In 1839, Charles Goodyear discovered a way of processing natural rubber, which is too stiff when cold and too soft when warm, in such a way as to make it elastic. This proved to have advantages for the manufacture of condoms; unlike the sheep's gut condoms, they could stretch and did not tear quickly when used. The rubber vulcanization process was patented by Goodyear in 1844. The first rubber condom was produced in 1855. The earliest rubber condoms had a seam and were as thick as a bicycle inner tube. Besides this type, small rubber condoms covering only the glans were often used in England and the United States. There was more risk of losing them and if the rubber ring was too tight, it would constrict the penis. This type of condom was the original "capote" (French for condom), perhaps because of its resemblance to a woman's bonnet worn at that time, also called a capote.

For many decades, rubber condoms were manufactured by wrapping strips of raw rubber around penis-shaped molds, then dipping the wrapped molds in a chemical solution to cure the rubber. In 1912, Polish-born inventor Julius Fromm developed a new, improved manufacturing technique for condoms: dipping glass molds into a raw rubber solution. Called cement dipping, this method required adding gasoline or benzene to the rubber to make it liquid. Around 1920 patent lawyer and vice-president of the United States Rubber Company Ernest Hopkinson invented a new technique of converting latex into rubber without a coagulant (demulsifier), which featured using water as a solvent and warm air to dry the solution, as well as optionally preserving liquid latex with ammonia. Condoms made this way, commonly called "latex" ones, required less labor to produce than cement-dipped rubber condoms, which had to be smoothed by rubbing and trimming. The use of water to suspend the rubber instead of gasoline and benzene eliminated the fire hazard previously associated with all condom factories. Latex condoms also performed better for the consumer: they were stronger and thinner than rubber condoms, and had a shelf life of five years (compared to three months for rubber).

Until the twenties, all condoms were individually hand-dipped by semi-skilled workers. Throughout the decade of the 1920s, advances in the automation of the condom assembly line were made. The first fully automated line was patented in 1930. Major condom manufacturers bought or leased conveyor systems, and small manufacturers were driven out of business. The skin condom, now significantly more expensive than the latex variety, became restricted to a niche high-end market.

1930 to present
In 1930 the Anglican Church's Lambeth Conference sanctioned the use of birth control by married couples. In 1931 the Federal Council of Churches in the U.S. issued a similar statement. The Roman Catholic Church responded by issuing the encyclical Casti connubii affirming its opposition to all contraceptives, a stance it has never reversed. In the 1930s, legal restrictions on condoms began to be relaxed. But during this period Fascist Italy and Nazi Germany increased restrictions on condoms (limited sales as disease preventatives were still allowed). During the Depression, condom lines by Schmid gained in popularity. Schmid still used the cement-dipping method of manufacture which had two advantages over the latex variety. Firstly, cement-dipped condoms could be safely used with oil-based lubricants. Secondly, while less comfortable, these older-style rubber condoms could be reused and so were more economical, a valued feature in hard times. More attention was brought to quality issues in the 1930s, and the U.S. Food and Drug Administration began to regulate the quality of condoms sold in the United States.

Throughout World War II, condoms were not only distributed to male U.S. military members, but also heavily promoted with films, posters, and lectures. European and Asian militaries on both sides of the conflict also provided condoms to their troops throughout the war, even Germany which outlawed all civilian use of condoms in 1941. In part because condoms were readily available, soldiers found a number of non-sexual uses for the devices, many of which continue to this day. After the war, condom sales continued to grow. From 1955 to 1965, 42% of Americans of reproductive age relied on condoms for birth control. In Britain from 1950 to 1960, 60% of married couples used condoms. The birth control pill became the world's most popular method of birth control in the years after its 1960 début, but condoms remained a strong second. The U.S. Agency for International Development pushed condom use in developing countries to help solve the "world population crises": by 1970 hundreds of millions of condoms were being used each year in India alone. (This number has grown in recent decades: in 2004, the government of India purchased 1.9 billion condoms for distribution at family planning clinics.) In the 1960s and 1970s quality regulations tightened, and more legal barriers to condom use were removed. In Ireland, legal condom sales were allowed for the first time in 1978. Advertising, however was one area that continued to have legal restrictions. In the late 1950s, the American National Association of Broadcasters banned condom advertisements from national television; this policy remained in place until 1979.

After it was discovered in the early 1980s that AIDS can be a sexually transmitted infection, the use of condoms was encouraged to prevent transmission of HIV. Despite opposition by some political, religious, and other figures, national condom promotion campaigns occurred in the U.S. and Europe. These campaigns increased condom use significantly.

Due to increased demand and greater social acceptance, condoms began to be sold in a wider variety of retail outlets, including in supermarkets and in discount department stores such as Walmart. Condom sales increased every year until 1994, when media attention to the AIDS pandemic began to decline. The phenomenon of decreasing use of condoms as disease preventatives has been called prevention fatigue or condom fatigue. Observers have cited condom fatigue in both Europe and North America. As one response, manufacturers have changed the tone of their advertisements from scary to humorous.

New developments continued to occur in the condom market, with the first polyurethane condom—branded Avanti and produced by the manufacturer of Durex—introduced in the 1990s. Worldwide condom use is expected to continue to grow: one study predicted that developing nations would need 18.6 billion condoms by 2015. , condoms are available inside prisons in Canada, most of the European Union, Australia, Brazil, Indonesia, South Africa, and the US states of Vermont (on 17 September 2013, the Californian Senate approved a bill for condom distribution inside the state's prisons, but the bill was not yet law at the time of approval).

The global condom market was estimated at US$9.2 billion in 2020.

Etymology and other terms
The term condom first appears in the early 18th century: early forms include condum (1706 and 1717), condon (1708) and cundum (1744). The word's etymology is unknown. In popular tradition, the invention and naming of the condom came to be attributed to an associate of England's King Charles II, one "Dr. Condom" or "Earl of Condom". There is however no evidence of the existence of such a person, and condoms had been used for over one hundred years before King Charles II acceded to the throne in 1660.

A variety of unproven Latin etymologies have been proposed, including condon (receptacle), condamina (house), and cumdum (scabbard or case). It has also been speculated to be from the Italian word guantone, derived from guanto, meaning glove. William E. Kruck wrote an article in 1981 concluding that, "As for the word 'condom', I need state only that its origin remains completely unknown, and there ends this search for an etymology." Modern dictionaries may also list the etymology as "unknown".

Other terms are also commonly used to describe condoms. In North America condoms are also commonly known as prophylactics, or rubbers. In Britain they may be called French letters or rubber johnnies. Additionally, condoms may be referred to using the manufacturer's name.

Society and culture
Some moral and scientific criticism of condoms exists despite the many benefits of condoms agreed on by scientific consensus and sexual health experts.

Condom usage is typically recommended for new couples who have yet to develop full trust in their partner with regard to STIs. Established couples on the other hand have few concerns about STIs, and can use other methods of birth control such as the pill, which does not act as a barrier to intimate sexual contact. Note that the polar debate with regard to condom usage is attenuated by the target group the argument is directed. Notably the age category and stable partner question are factors, as well as the distinction between heterosexual and homosexuals, who have different kinds of sex and have different risk consequences and factors.

Among the prime objections to condom usage is the blocking of erotic sensation, or the intimacy that barrier-free sex provides. As the condom is held tightly to the skin of the penis, it diminishes the delivery of stimulation through rubbing and friction. Condom proponents claim this has the benefit of making sex last longer, by diminishing sensation and delaying male ejaculation. Those who promote condom-free heterosexual sex (slang: "bareback") claim that the condom puts a barrier between partners, diminishing what is normally a highly sensual, intimate, and spiritual connection between partners.

Religious
The United Church of Christ (UCC), a Reformed denomination of the Congregationalist tradition, promotes the distribution of condoms in churches and faith-based educational settings. Michael Shuenemeyer, a UCC minister, has stated that "The practice of safer sex is a matter of life and death. People of faith make condoms available because we have chosen life so that we and our children may live."

On the other hand, the Roman Catholic Church opposes all kinds of sexual acts outside of marriage, as well as any sexual act in which the chance of successful conception has been reduced by direct and intentional acts (for example, surgery to prevent conception) or foreign objects (for example, condoms).

The use of condoms to prevent STI transmission is not specifically addressed by Catholic doctrine, and is currently a topic of debate among theologians and high-ranking Catholic authorities. A few, such as Belgian Cardinal Godfried Danneels, believe the Catholic Church should actively support condoms used to prevent disease, especially serious diseases such as AIDS. However, the majority view—including all statements from the Vatican—is that condom-promotion programs encourage promiscuity, thereby actually increasing STI transmission. This view was most recently reiterated in 2009 by Pope Benedict XVI.

The Roman Catholic Church is the largest organized body of any world religion. The church has hundreds of programs dedicated to fighting the AIDS epidemic in Africa, but its opposition to condom use in these programs has been highly controversial.

In a November 2011 interview, Pope Benedict XVI discussed for the first time the use of condoms to prevent STI transmission. He said that the use of a condom can be justified in a few individual cases if the purpose is to reduce the risk of an HIV infection. He gave as an example male prostitutes. There was some confusion at first whether the statement applied only to homosexual prostitutes and thus not to heterosexual intercourse at all. However, Federico Lombardi, spokesman for the Vatican, clarified that it applied to heterosexual and transsexual prostitutes, whether male or female, as well. He did, however, also clarify that the Vatican's principles on sexuality and contraception had not been changed.

Scientific and environmental
More generally, some scientific researchers have expressed objective concern over certain ingredients sometimes added to condoms, notably talc and nitrosamines. Dry dusting powders are applied to latex condoms before packaging to prevent the condom from sticking to itself when rolled up. Previously, talc was used by most manufacturers, but cornstarch is currently the most popular dusting powder. Although rare during normal use, talc is known to be potentially irritant to mucous membranes (such as in the vagina). Cornstarch is generally believed to be safe; however, some researchers have raised concerns over its use as well.

Nitrosamines, which are potentially carcinogenic in humans, are believed to be present in a substance used to improve elasticity in latex condoms. A 2001 review stated that humans regularly receive 1,000 to 10,000 times greater nitrosamine exposure from food and tobacco than from condom use and concluded that the risk of cancer from condom use is very low. However, a 2004 study in Germany detected nitrosamines in 29 out of 32 condom brands tested, and concluded that exposure from condoms might exceed the exposure from food by 1.5- to 3-fold. In addition, the large-scale use of disposable condoms has resulted in concerns over their environmental impact via littering and in landfills, where they can eventually wind up in wildlife environments if not incinerated or otherwise permanently disposed of first. Polyurethane condoms in particular, given they are a form of plastic, are not biodegradable, and latex condoms take a very long time to break down. Experts, such as AVERT, recommend condoms be disposed of in a garbage receptacle, as flushing them down the toilet (which some people do) may cause plumbing blockages and other problems. Furthermore, the plastic and foil wrappers condoms are packaged in are also not biodegradable. However, the benefits condoms offer are widely considered to offset their small landfill mass. Frequent condom or wrapper disposal in public areas such as a parks have been seen as a persistent litter problem.

While biodegradable, latex condoms damage the environment when disposed of improperly. According to the Ocean Conservancy, condoms, along with certain other types of trash, cover the coral reefs and smother sea grass and other bottom dwellers. The United States Environmental Protection Agency also has expressed concerns that many animals might mistake the litter for food.

Cultural barriers to use
In much of the Western world, the introduction of the pill in the 1960s was associated with a decline in condom use. In Japan, oral contraceptives were not approved for use until September 1999, and even then access was more restricted than in other industrialized nations. Perhaps because of this restricted access to hormonal contraception, Japan has the highest rate of condom usage in the world: in 2008, 80% of contraceptive users relied on condoms.

Cultural attitudes toward gender roles, contraception, and sexual activity vary greatly around the world, and range from extremely conservative to extremely liberal. But in places where condoms are misunderstood, mischaracterised, demonised, or looked upon with overall cultural disapproval, the prevalence of condom use is directly affected. In less-developed countries and among less-educated populations, misperceptions about how disease transmission and conception work negatively affect the use of condoms; additionally, in cultures with more traditional gender roles, women may feel uncomfortable demanding that their partners use condoms.

As an example, Latino immigrants in the United States often face cultural barriers to condom use. A study on female HIV prevention published in the Journal of Sex Health Research asserts that Latino women often lack the attitudes needed to negotiate safe sex due to traditional gender-role norms in the Latino community, and may be afraid to bring up the subject of condom use with their partners. Women who participated in the study often reported that because of the general machismo subtly encouraged in Latino culture, their male partners would be angry or possibly violent at the woman's suggestion that they use condoms. A similar phenomenon has been noted in a survey of low-income American black women; the women in this study also reported a fear of violence at the suggestion to their male partners that condoms be used.

A telephone survey conducted by Rand Corporation and Oregon State University, and published in the Journal of Acquired Immune Deficiency Syndromes showed that belief in AIDS conspiracy theories among United States black men is linked to rates of condom use. As conspiracy beliefs about AIDS grow in a given sector of these black men, consistent condom use drops in that same sector. Female use of condoms was not similarly affected.

In the African continent, condom promotion in some areas has been impeded by anti-condom campaigns by some Muslim and Catholic clerics. Among the Maasai in Tanzania, condom use is hampered by an aversion to "wasting" sperm, which is given sociocultural importance beyond reproduction. Sperm is believed to be an "elixir" to women and to have beneficial health effects. Maasai women believe that, after conceiving a child, they must have sexual intercourse repeatedly so that the additional sperm aids the child's development. Frequent condom use is also considered by some Maasai to cause impotence. Some women in Africa believe that condoms are "for prostitutes" and that respectable women should not use them. A few clerics even promote the lie that condoms are deliberately laced with HIV. In the United States, possession of many condoms has been used by police to accuse women of engaging in prostitution. The Presidential Advisory Council on HIV/AIDS has condemned this practice and there are efforts to end it.

Middle-Eastern couples who have not had children, because of the strong desire and social pressure to establish fertility as soon as possible within marriage, rarely use condoms.

In 2017, India restricted TV advertisements for condoms to between the hours of 10 pm to 6 am. Family planning advocates were against this, saying it was liable to "undo decades of progress on sexual and reproductive health".

Major manufacturers
One analyst described the size of the condom market as something that "boggles the mind". Numerous small manufacturers, nonprofit groups, and government-run manufacturing plants exist around the world. Within the condom market, there are several major contributors, among them both for-profit businesses and philanthropic organizations. Most large manufacturers have ties to the business that reach back to the end of the 19th century.

Research
A spray-on condom made of latex is intended to be easier to apply and more successful in preventing the transmission of diseases. , the spray-on condom was not going to market because the drying time could not be reduced below two to three minutes.

The Invisible Condom, developed at Université Laval in Quebec, Canada, is a gel that hardens upon increased temperature after insertion into the vagina or rectum. In the lab, it has been shown to effectively block HIV and herpes simplex virus. The barrier breaks down and liquefies after several hours. , the invisible condom is in the clinical trial phase, and has not yet been approved for use.

Also developed in 2005 is a condom treated with an erectogenic compound. The drug-treated condom is intended to help the wearer maintain an erection, which should also help reduce slippage. If approved, the condom would be marketed under the Durex brand. , it was still in clinical trials. In 2009, Ansell Healthcare, the makers of Lifestyle condoms, introduced the X2 condom lubricated with "Excite Gel" which contains the amino acid L-arginine and is intended to improve the strength of the erectile response.

In March 2013, philanthropist Bill Gates offered US$100,000 grants through his foundation for a condom design that "significantly preserves or enhances pleasure" to encourage more males to adopt the use of condoms for safer sex. The grant information stated: "The primary drawback from the male perspective is that condoms decrease pleasure as compared to no condom, creating a trade-off that many men find unacceptable, particularly given that the decisions about use must be made just prior to intercourse. Is it possible to develop a product without this stigma, or better, one that is felt to enhance pleasure?" In November of the same year, 11 research teams were selected to receive the grant money.

title: Condom


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Country code

A country code is a short alphanumeric identification code for countries and dependent areas. Its primary use is in data processing and communications. Several identification systems have been developed.

The term country code frequently refers to ISO 3166-1 alpha-2, as well as the telephone country code, which is embodied in the E.164 recommendation by the International Telecommunication Union (ITU).

ISO 3166-1
The standard ISO 3166-1 defines short identification codes for most countries and dependent areas:

 * ISO 3166-1 alpha-2: two-letter code
 * ISO 3166-1 alpha-3: three-letter code
 * ISO 3166-1 numeric: three-digit code

The two-letter codes are used as the basis for other codes and applications, for example,

 * for ISO 4217 currency codes
 * with deviations, for country code top-level domain names (ccTLDs) on the Internet: list of Internet TLDs.

Other applications are defined in ISO 3166-1 alpha-2.

ITU country codes
In telecommunication, a country code, or international subscriber dialing (ISD) code, is a telephone number prefix used in international direct dialing (IDD) and for destination routing of telephone calls to a country other than the caller's. A country or region with an autonomous telephone administration must apply for membership in the International Telecommunication Union (ITU) to participate in the international public switched telephone network (PSTN). County codes are defined by the ITU-T section of the ITU in standards E.123 and E.164.

Country codes constitute the international telephone numbering plan, and are dialed only when calling a telephone number in another country. They are dialed before the national telephone number. International calls require at least one additional prefix to be dialing before the country code, to connect the call to international circuits, the international call prefix. When printing telephone numbers this is indicated by a plus-sign (+) in front of a complete international telephone number, per recommendation E164 by the ITU.

Other country codes

 * European Union:
 * Before the 2004 EU enlargement the EU used the UN Road Traffic Conventions license plate codes. Since then, it has used the ISO 3166-1 alpha-2 code, but with two modifications:
 * EL for Greece (instead of GR)
 * (formerly) UK for United Kingdom (instead of GB)
 * The Nomenclature des unités territoriales statistiques (Nomenclature of territorial units for statistics, NUTS) of the European Union, mostly focusing on subdivisions of the EU member states
 * FIFA (Fédération Internationale de Football Association) assigns a three-letter code (dubbed FIFA Trigramme) to each of its member and non-member countries: List of FIFA country codes
 * Federal Information Processing Standard (FIPS) 10-4 defined two-letter codes used by the U.S. government and in the CIA World Factbook: list of FIPS country codes. On September 2, 2008, FIPS 10-4 was one of ten standards withdrawn by NIST as a Federal Information Processing Standard.
 * The Bureau of Transportation Statistics, part of the United States Department of Transportation (US DOT), maintains its own list of codes, so-called World Area Codes (WAC), for state and country codes.
 * GOST 7.67: country codes in Cyrillic from the GOST standards committee
 * From the International Civil Aviation Organization (ICAO):
 * The national prefixes used in aircraft registration numbers
 * Location prefixes in four-character ICAO airport codes
 * International Olympic Committee (IOC) three-letter codes used in sporting events: list of IOC country codes
 * From the International Telecommunication Union (ITU):
 * the E.212 mobile country codes (MCC), for mobile/wireless phone addresses,
 * the first few characters of call signs of radio stations (maritime, aeronautical, amateur radio, broadcasting, and so on) define the country: the ITU prefix,
 * ITU letter codes for member-countries,
 * ITU prefix - amateur and experimental stations - The International Telecommunication Union (ITU) assigns national telecommunication prefixes for amateur and experimental radio use, so that operators can be identified by their country of origin. These prefixes are legally administered by the national entity to which prefix ranges are assigned.
 * Three-digit codes used to identify countries in maritime mobile radio transmissions, known as maritime identification digits
 * License plates for automobiles:
 * Under the 1949 and 1968 United Nations Road Traffic Conventions (distinguishing signs of vehicles in international traffic): List of international license plate codes.
 * Diplomatic license plates in the United States, assigned by the U.S. State Department.
 * North Atlantic Treaty Organization (NATO) used two-letter codes of its own: list of NATO country codes. They were largely borrowed from the FIPS 10-4 codes mentioned below. In 2003 the eighth edition of the Standardisation Agreement (STANAG) adopted the ISO 3166 three-letter codes with one exception (the code for Macedonia). With the ninth edition, NATO is transitioning to four- and six-letter codes based on ISO 3166 with a few exceptions and additions
 * United Nations Development Programme (UNDP) also has its own list of trigram country codes
 * World Intellectual Property Organization (WIPO): WIPO ST.3 gives two-letter codes to countries and regional intellectual property organizations
 * World Meteorological Organization (WMO) maintains a list of country codes, used in reporting meteorological observations
 * UIC (the International Union of Railways): UIC Country Codes

The developers of ISO 3166 intended that in time it would replace other coding systems.

Other codings
Country identities may be encoded in the following coding systems:
 * The initial digits of International Standard Book Numbers (ISBN) are group identifiers for countries, areas, or language regions.
 * The first three digits of GS1 Company Prefixes used to identify products, for example, in barcodes, designate (national) numbering agencies.

Lists of country codes by country
A - B - C - D–E - F - G - H–I - J–K - L - M - N - O–Q - R - S - T - U–Z

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Country code


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Cladistics

Cladistics is an approach to biological classification in which organisms are categorized in groups ("clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived characteristics (synapomorphies) that are not present in more distant groups and ancestors. However, from an empirical perspective, common ancestors are inferences based on a cladistic hypothesis of relationships of taxa whose character states can be observed. Theoretically, a last common ancestor and all its descendants constitute a (minimal) clade. Importantly, all descendants stay in their overarching ancestral clade. For example, if the terms worms or fishes were used within a strict cladistic framework, these terms would include humans. Many of these terms are normally used paraphyletically, outside of cladistics, e.g. as a 'grade', which are fruitless to precisely delineate, especially when including extinct species. Radiation results in the generation of new subclades by bifurcation, but in practice sexual hybridization may blur very closely related groupings.

As a hypothesis, a clade can be rejected only if some groupings were explicitly excluded. It may then be found that the excluded group did actually descend from the last common ancestor of the group, and thus emerged within the group. ("Evolved from" is misleading, because in cladistics all descendants stay in the ancestral group). To keep only valid clades, upon finding that the group is paraphyletic this way, either such excluded groups should be granted to the clade, or the group should be abolished.

Branches down to the divergence to the next significant (e.g. extant) sister are considered stem-groupings of the clade, but in principle each level stands on its own, to be assigned a unique name. For a fully bifurcated tree, adding a group to a tree also adds an additional (named) clade, and a new level on that branch. Specifically, also extinct groups are always put on a side-branch, not distinguishing whether an actual ancestor of other groupings was found.

The techniques and nomenclature of cladistics have been applied to disciplines other than biology. (See phylogenetic nomenclature.)

Cladistics findings are posing a difficulty for taxonomy, where the rank and (genus-)naming of established groupings may turn out to be inconsistent.

Cladistics is now the most commonly used method to classify organisms.

History

The original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics (also the title of his 1966 book); but the terms "cladistics" and "clade" were popularized by other researchers. Cladistics in the original sense refers to a particular set of methods used in phylogenetic analysis, although it is now sometimes used to refer to the whole field.

What is now called the cladistic method appeared as early as 1901 with a work by Peter Chalmers Mitchell for birds and subsequently by Robert John Tillyard (for insects) in 1921, and W. Zimmermann (for plants) in 1943. The term "clade" was introduced in 1958 by Julian Huxley after having been coined by Lucien Cuénot in 1940, "cladogenesis" in 1958, "cladistic" by Arthur Cain and Harrison in 1960, "cladist" (for an adherent of Hennig's school) by Ernst Mayr in 1965, and "cladistics" in 1966. Hennig referred to his own approach as "phylogenetic systematics". From the time of his original formulation until the end of the 1970s, cladistics competed as an analytical and philosophical approach to systematics with phenetics and so-called evolutionary taxonomy. Phenetics was championed at this time by the numerical taxonomists Peter Sneath and Robert Sokal, and evolutionary taxonomy by Ernst Mayr.

Originally conceived, if only in essence, by Willi Hennig in a book published in 1950, cladistics did not flourish until its translation into English in 1966 (Lewin 1997). Today, cladistics is the most popular method for inferring phylogenetic trees from morphological data.

In the 1990s, the development of effective polymerase chain reaction techniques allowed the application of cladistic methods to biochemical and molecular genetic traits of organisms, vastly expanding the amount of data available for phylogenetics. At the same time, cladistics rapidly became popular in evolutionary biology, because computers made it possible to process large quantities of data about organisms and their characteristics.

Methodology
The cladistic method interprets each shared character state transformation as a potential piece of evidence for grouping. Synapomorphies (shared, derived character states) are viewed as evidence of grouping, while symplesiomorphies (shared ancestral character states) are not. The outcome of a cladistic analysis is a cladogram – a tree-shaped diagram (dendrogram) that is interpreted to represent the best hypothesis of phylogenetic relationships. Although traditionally such cladograms were generated largely on the basis of morphological characters and originally calculated by hand, genetic sequencing data and computational phylogenetics are now commonly used in phylogenetic analyses, and the parsimony criterion has been abandoned by many phylogeneticists in favor of more "sophisticated" but less parsimonious evolutionary models of character state transformation. Cladists contend that these models are unjustified because there is no evidence that they recover more "true" or "correct" results from actual empirical data sets

Every cladogram is based on a particular dataset analyzed with a particular method. Datasets are tables consisting of molecular, morphological, ethological and/or other characters and a list of operational taxonomic units (OTUs), which may be genes, individuals, populations, species, or larger taxa that are presumed to be monophyletic and therefore to form, all together, one large clade; phylogenetic analysis infers the branching pattern within that clade. Different datasets and different methods, not to mention violations of the mentioned assumptions, often result in different cladograms. Only scientific investigation can show which is more likely to be correct.

Until recently, for example, cladograms like the following have generally been accepted as accurate representations of the ancestral relations among turtles, lizards, crocodilians, and birds:

If this phylogenetic hypothesis is correct, then the last common ancestor of turtles and birds, at the branch near the ▼ lived earlier than the last common ancestor of lizards and birds, near the ♦. Most molecular evidence, however, produces cladograms more like this:

If this is accurate, then the last common ancestor of turtles and birds lived later than the last common ancestor of lizards and birds. Since the cladograms show two mutually exclusive hypotheses to describe the evolutionary history, at most one of them is correct.

The cladogram to the right represents the current universally accepted hypothesis that all primates, including strepsirrhines like the lemurs and lorises, had a common ancestor all of whose descendants are or were primates, and so form a clade; the name Primates is therefore recognized for this clade. Within the primates, all anthropoids (monkeys, apes, and humans) are hypothesized to have had a common ancestor all of whose descendants are or were anthropoids, so they form the clade called Anthropoidea. The "prosimians", on the other hand, form a paraphyletic taxon. The name Prosimii is not used in phylogenetic nomenclature, which names only clades; the "prosimians" are instead divided between the clades Strepsirhini and Haplorhini, where the latter contains Tarsiiformes and Anthropoidea.

Lemurs and tarsiers may have looked closely related to humans, in the sense of being close on the evolutionary tree to humans. However, from the perspective of a tarsier, humans and lemurs would have looked close, in the exact same sense. Cladistics forces a neutral perspective, treating all branches (extant or extinct) in the same manner. It also forces one to try to make statements, and honestly take into account findings, about the exact historic relationships between the groups.

Terminology for character states
The following terms, coined by Hennig, are used to identify shared or distinct character states among groups:
 * A plesiomorphy ("close form") or ancestral state is a character state that a taxon has retained from its ancestors. When two or more taxa that are not nested within each other share a plesiomorphy, it is a symplesiomorphy (from syn-, "together"). Symplesiomorphies do not mean that the taxa that exhibit that character state are necessarily closely related. For example, Reptilia is traditionally characterized by (among other things) being cold-blooded (i.e., not maintaining a constant high body temperature), whereas birds are warm-blooded. Since cold-bloodedness is a plesiomorphy, inherited from the common ancestor of traditional reptiles and birds, and thus a symplesiomorphy of turtles, snakes and crocodiles (among others), it does not mean that turtles, snakes and crocodiles form a clade that excludes the birds.
 * An apomorphy ("separate form") or derived state is an innovation. It can thus be used to diagnose a clade – or even to help define a clade name in phylogenetic nomenclature. Features that are derived in individual taxa (a single species or a group that is represented by a single terminal in a given phylogenetic analysis) are called autapomorphies (from auto-, "self"). Autapomorphies express nothing about relationships among groups; clades are identified (or defined) by synapomorphies (from syn-, "together"). For example, the possession of digits that are homologous with those of Homo sapiens is a synapomorphy within the vertebrates. The tetrapods can be singled out as consisting of the first vertebrate with such digits homologous to those of Homo sapiens together with all descendants of this vertebrate (an apomorphy-based phylogenetic definition). Importantly, snakes and other tetrapods that do not have digits are nonetheless tetrapods: other characters, such as amniotic eggs and diapsid skulls, indicate that they descended from ancestors that possessed digits which are homologous with ours.
 * A character state is homoplastic or "an instance of homoplasy" if it is shared by two or more organisms but is absent from their common ancestor or from a later ancestor in the lineage leading to one of the organisms. It is therefore inferred to have evolved by convergence or reversal. Both mammals and birds are able to maintain a high constant body temperature (i.e., they are warm-blooded). However, the accepted cladogram explaining their significant features indicates that their common ancestor is in a group lacking this character state, so the state must have evolved independently in the two clades. Warm-bloodedness is separately a synapomorphy of mammals (or a larger clade) and of birds (or a larger clade), but it is not a synapomorphy of any group including both these clades. Hennig's Auxiliary Principle states that shared character states should be considered evidence of grouping unless they are contradicted by the weight of other evidence; thus, homoplasy of some feature among members of a group may only be inferred after a phylogenetic hypothesis for that group has been established.

The terms plesiomorphy and apomorphy are relative; their application depends on the position of a group within a tree. For example, when trying to decide whether the tetrapods form a clade, an important question is whether having four limbs is a synapomorphy of the earliest taxa to be included within Tetrapoda: did all the earliest members of the Tetrapoda inherit four limbs from a common ancestor, whereas all other vertebrates did not, or at least not homologously? By contrast, for a group within the tetrapods, such as birds, having four limbs is a plesiomorphy. Using these two terms allows a greater precision in the discussion of homology, in particular allowing clear expression of the hierarchical relationships among different homologous features.

It can be difficult to decide whether a character state is in fact the same and thus can be classified as a synapomorphy, which may identify a monophyletic group, or whether it only appears to be the same and is thus a homoplasy, which cannot identify such a group. There is a danger of circular reasoning: assumptions about the shape of a phylogenetic tree are used to justify decisions about character states, which are then used as evidence for the shape of the tree. Phylogenetics uses various forms of parsimony to decide such questions; the conclusions reached often depend on the dataset and the methods. Such is the nature of empirical science, and for this reason, most cladists refer to their cladograms as hypotheses of relationship. Cladograms that are supported by a large number and variety of different kinds of characters are viewed as more robust than those based on more limited evidence.

Terminology for taxa
Mono-, para- and polyphyletic taxa can be understood based on the shape of the tree (as done above), as well as based on their character states. These are compared in the table below.

Criticism
Cladistics, either generally or in specific applications, has been criticized from its beginnings. Decisions as to whether particular character states are homologous, a precondition of their being synapomorphies, have been challenged as involving circular reasoning and subjective judgements. Of course, the potential unreliability of evidence is a problem for any systematic method, or for that matter, for any empirical scientific endeavor at all.

Transformed cladistics arose in the late 1970s in an attempt to resolve some of these problems by removing a priori assumptions about phylogeny from cladistic analysis, but it has remained unpopular.

Ancestors
The cladistic method does not identify fossil species as actual ancestors of a clade. Instead, fossil taxa are identified as belonging to separate extinct branches. While a fossil species could be the actual ancestor of a clade, there is no way to know that. Therefore, a more conservative hypothesis is that the fossil taxon is related to other fossil and extant taxa, as implied by the pattern of shared apomorphic features.

Extinction status
An otherwise extinct group with any extant descendants, is not considered (literally) extinct, and for instance does not have a date of extinction.

Hybridization, interbreeding
Anything having to do with biology and sex is complicated and messy, and cladistics is no exception. Many species reproduce sexually, and are capable of interbreeding for millions of years. Worse, during such a period, many branches may have radiated, and it may take hundreds of millions of years for them to have whittled down to just two. Only then one can theoretically assign proper last common ancestors of groupings which do not inadvertently include earlier branches. The process of true cladistic bifurcation can thus take a much more extended time than one is usually aware of. In practice, for recent radiations, cladistically guided findings only give a coarse impression of the complexity. A more detailed account will give details about fractions of introgressions between groupings, and even geographic variations thereof. This has been used as an argument for the use of paraphyletic groupings, but typically other reasons are quoted.

Horizontal gene transfer
Horizontal gene transfer is the mobility of genetic info between different organisms that can have immediate or delayed effects for the reciprocal host. There are several processes in nature which can cause horizontal gene transfer. This does typically not directly interfere with ancestry of the organism, but can complicate the determination of that ancestry. On another level, one can map the horizontal gene transfer processes, by determining the phylogeny of the individual genes using cladistics.

Naming stability
If there is unclarity in mutual relationships, there are a lot of possible trees. Assigning names to each possible clade may not be prudent. Furthermore, established names are discarded in cladistics, or alternatively carry connotations which may no longer hold, such as when additional groups are found to have emerged in them. Naming changes are the direct result of changes in the recognition of mutual relationships, which often is still in flux, especially for extinct species. Hanging on to older naming and/or connotations is counter-productive, as they typically do not reflect actual mutual relationships precisely at all. E.g. Archaea, Asgard archaea, protists, slime molds, worms, invertebrata, fishes, reptilia, monkeys, Ardipithecus, Australopithecus, Homo erectus all contain Homo sapiens cladistically, in their sensu lato meaning. For originally extinct stem groups, sensu lato generally means generously keeping previously included groups, which then may come to include even living species. A pruned sensu stricto meaning is often adopted instead, but the group would need to be restricted to a single branch on the stem. Other branches then get their own name and level. This is commensurate to the fact that more senior stem branches are in fact closer related to the resulting group than the more basal stem branches; that those stem branches only may have lived for a short time does not affect that assessment in cladistics.

In disciplines other than biology
The comparisons used to acquire data on which cladograms can be based are not limited to the field of biology. Any group of individuals or classes that are hypothesized to have a common ancestor, and to which a set of common characteristics may or may not apply, can be compared pairwise. Cladograms can be used to depict the hypothetical descent relationships within groups of items in many different academic realms. The only requirement is that the items have characteristics that can be identified and measured.

Anthropology and archaeology: Cladistic methods have been used to reconstruct the development of cultures or artifacts using groups of cultural traits or artifact features.

Comparative mythology and folktale use cladistic methods to reconstruct the protoversion of many myths. Mythological phylogenies constructed with mythemes clearly support low horizontal transmissions (borrowings), historical (sometimes Palaeolithic) diffusions and punctuated evolution. They also are a powerful way to test hypotheses about cross-cultural relationships among folktales.

Literature: Cladistic methods have been used in the classification of the surviving manuscripts of the Canterbury Tales, and the manuscripts of the Sanskrit Charaka Samhita.

Historical linguistics: Cladistic methods have been used to reconstruct the phylogeny of languages using linguistic features. This is similar to the traditional comparative method of historical linguistics, but is more explicit in its use of parsimony and allows much faster analysis of large datasets (computational phylogenetics).

Textual criticism or stemmatics: Cladistic methods have been used to reconstruct the phylogeny of manuscripts of the same work (and reconstruct the lost original) using distinctive copying errors as apomorphies. This differs from traditional historical-comparative linguistics in enabling the editor to evaluate and place in genetic relationship large groups of manuscripts with large numbers of variants that would be impossible to handle manually. It also enables parsimony analysis of contaminated traditions of transmission that would be impossible to evaluate manually in a reasonable period of time.

Astrophysics infers the history of relationships between galaxies to create branching diagram hypotheses of galaxy diversification.

title: Cladistics


text:
Calendar

A calendar is a system of organizing days. This is done by giving names to periods of time, typically days, weeks, months and years. A date is the designation of a single and specific day within such a system. A calendar is also a physical record (often paper) of such a system. A calendar can also mean a list of planned events, such as a court calendar, or a partly or fully chronological list of documents, such as a calendar of wills.

Periods in a calendar (such as years and months) are usually, though not necessarily, synchronized with the cycle of the sun or the moon. The most common type of pre-modern calendar was the lunisolar calendar, a lunar calendar that occasionally adds one intercalary month to remain synchronized with the solar year over the long term.

Etymology
The term calendar is taken from kalendae, the term for the first day of the month in the Roman calendar, related to the verb calare 'to call out', referring to the "calling" of the new moon when it was first seen. Latin calendarium meant 'account book, register' (as accounts were settled and debts were collected on the calends of each month). The Latin term was adopted in Old French as calendier and from there in Middle English as calender by the 13th century (the spelling calendar is early modern).

History

The course of the Sun and the Moon are the most salient regularly recurring natural events useful for timekeeping, and in pre-modern societies around the world lunation and the year were most commonly used as time units. Nevertheless, the Roman calendar contained remnants of a very ancient pre-Etruscan 10-month solar year.

The first recorded physical calendars, dependent on the development of writing in the Ancient Near East, are the Bronze Age Egyptian and Sumerian calendars.

During the Vedic period India developed a sophisticated timekeeping methodology and calendars for Vedic rituals. According to Yukio Ohashi, the Vedanga calendar in ancient India was based on astronomical studies during the Vedic Period and was not derived from other cultures.

A large number of calendar systems in the Ancient Near East were based on the Babylonian calendar dating from the Iron Age, among them the calendar system of the Persian Empire, which in turn gave rise to the Zoroastrian calendar and the Hebrew calendar.

A great number of Hellenic calendars were developed in Classical Greece, and during the Hellenistic period they gave rise to the ancient Roman calendar and to various Hindu calendars.

Calendars in antiquity were lunisolar, depending on the introduction of intercalary months to align the solar and the lunar years. This was mostly based on observation, but there may have been early attempts to model the pattern of intercalation algorithmically, as evidenced in the fragmentary 2nd-century Coligny calendar.

The Roman calendar was reformed by Julius Caesar in 46 BC. His "Julian" calendar was no longer dependent on the observation of the new moon, but followed an algorithm of introducing a leap day every four years. This created a dissociation of the calendar month from lunation. The Gregorian calendar, introduced in 1582, corrected most of the remaining difference between the Julian calendar and the solar year.

The Islamic calendar is based on the prohibition of intercalation (nasi') by Muhammad, in Islamic tradition dated to a sermon given on 9 Dhu al-Hijjah AH 10 (Julian date: 6 March 632). This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year.

There have been several modern proposals for reform of the modern calendar, such as the World Calendar, the International Fixed Calendar, the Holocene calendar, and the Hanke–Henry Permanent Calendar. Such ideas are mooted from time to time, but have failed to gain traction because of the loss of continuity and the massive upheaval that implementing them would involve, as well as their effect on cycles of religious activity.

Systems

A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.

The simplest calendar system just counts time periods from a reference date. This applies for the Julian day or Unix Time. Virtually the only possible variation is using a different reference date, in particular, one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction.

Other calendars have one (or multiple) larger units of time.

Calendars that contain one level of cycles:
 * week and weekday – this system (without year, the week number keeps on increasing) is not very common
 * year and ordinal date within the year, e.g., the ISO 8601 ordinal date system

Calendars with two levels of cycles:
 * year, month, and day – most systems, including the Gregorian calendar (and its very similar predecessor, the Julian calendar), the Islamic calendar, the Solar Hijri calendar and the Hebrew calendar
 * year, week, and weekday – e.g., the ISO week date

Cycles can be synchronized with periodic phenomena:
 * Lunar calendars are synchronized to the motion of the Moon (lunar phases); an example is the Islamic calendar.
 * Solar calendars are based on perceived seasonal changes synchronized to the apparent motion of the Sun; an example is the Persian calendar.
 * Lunisolar calendars are based on a combination of both solar and lunar reckonings; examples include the traditional calendar of China, the Hindu calendar in India and Nepal, and the Hebrew calendar.
 * The week cycle is an example of one that is not synchronized to any external phenomenon (although it may have been derived from lunar phases, beginning anew every month).

Very commonly a calendar includes more than one type of cycle or has both cyclic and non-cyclic elements.

Most calendars incorporate more complex cycles. For example, the vast majority of them track years, months, weeks and days. The seven-day week is practically universal, though its use varies. It has run uninterrupted for millennia.

Solar
Solar calendars assign a date to each solar day. A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day. Other types of calendar may also use a solar day.

The Egyptians appear to have been the first to develop a solar calendar, using as a fixed point the annual sunrise reappearance of the Dog Star—Sirius, or Sothis—in the eastern sky, which coincided with the annual flooding of the Nile River. They built a calendar with 365 days, divided into 12 months of 30 days each, with 5 extra days at the end of the year. However, they did not include the extra bit of time in each year, and this caused their calendar to slowly become inaccurate.

Lunar
Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each lunar phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year, a purely lunar calendar quickly drifts against the seasons, which do not vary much near the equator. It does, however, stay constant with respect to other phenomena, notably tides. An example is the Islamic calendar. Alexander Marshack, in a controversial reading, believed that marks on a bone baton (c. 25,000 BC) represented a lunar calendar. Other marked bones may also represent lunar calendars. Similarly, Michael Rappenglueck believes that marks on a 15,000-year-old cave painting represent a lunar calendar.

Lunisolar
A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons. Prominent examples of lunisolar calendar are Hindu calendar and Buddhist calendar that are popular in South Asia and Southeast Asia. Another example is the Hebrew calendar, which uses a 19-year cycle.

Subdivisions
Nearly all calendar systems group consecutive days into "months" and also into "years". In a solar calendar a year approximates Earth's tropical year (that is, the time it takes for a complete cycle of seasons), traditionally used to facilitate the planning of agricultural activities. In a lunar calendar, the month approximates the cycle of the moon phase. Consecutive days may be grouped into other periods such as the week.

Because the number of days in the tropical year is not a whole number, a solar calendar must have a different number of days in different years. This may be handled, for example, by adding an extra day in leap years. The same applies to months in a lunar calendar and also the number of months in a year in a lunisolar calendar. This is generally known as intercalation. Even if a calendar is solar, but not lunar, the year cannot be divided entirely into months that never vary in length.

Cultures may define other units of time, such as the week, for the purpose of scheduling regular activities that do not easily coincide with months or years. Many cultures use different baselines for their calendars' starting years. Historically, several countries have based their calendars on regnal years, a calendar based on the reign of their current sovereign. For example, the year 2006 in Japan is year 18 Heisei, with Heisei being the era name of Emperor Akihito.

Arithmetical and astronomical

An astronomical calendar is based on ongoing observation; examples are the religious Islamic calendar and the old religious Jewish calendar in the time of the Second Temple. Such a calendar is also referred to as an observation-based calendar. The advantage of such a calendar is that it is perfectly and perpetually accurate. The disadvantage is that working out when a particular date would occur is difficult.

An arithmetic calendar is one that is based on a strict set of rules; an example is the current Jewish calendar. Such a calendar is also referred to as a rule-based calendar. The advantage of such a calendar is the ease of calculating when a particular date occurs. The disadvantage is imperfect accuracy. Furthermore, even if the calendar is very accurate, its accuracy diminishes slowly over time, owing to changes in Earth's rotation. This limits the lifetime of an accurate arithmetic calendar to a few thousand years. After then, the rules would need to be modified from observations made since the invention of the calendar.

Other variants
The early Roman calendar, created during the reign of Romulus, lumped the 61 days of the winter period them together as simply "winter." Over time, this period became January and February; through further changes over time (including the creation of the Julian calendar) this calendar became the modern Gregorian calendar, introduced in the 1570s.

Usage
The primary practical use of a calendar is to identify days: to be informed about or to agree on a future event and to record an event that has happened. Days may be significant for agricultural, civil, religious, or social reasons. For example, a calendar provides a way to determine when to start planting or harvesting, which days are religious or civil holidays, which days mark the beginning and end of business accounting periods, and which days have legal significance, such as the day taxes are due or a contract expires. Also, a calendar may, by identifying a day, provide other useful information about the day such as its season.

Calendars are also used as part of a complete timekeeping system: date and time of day together specify a moment in time. In the modern world, timekeepers can show time, date, and weekday. Some may also show the lunar phase.

Gregorian
The Gregorian calendar is the de facto international standard and is used almost everywhere in the world for civil purposes. The widely-used solar aspect is a cycle of leap days in a 400-year cycle designed to keep the duration of the year aligned with the solar year. There is a lunar aspect which approximates the position of the moon during the year, and is used in the calculation of the date of Easter. Each Gregorian year has either 365 or 366 days (the leap day being inserted as 29 February), amounting to an average Gregorian year of 365.2425 days (compared to a solar year of 365.2422 days).

The calendar was introduced in 1582 as a refinement to the Julian calendar, which had been in use throughout the European Middle Ages, amounting to a 0.002% correction in the length of the year. During the Early Modern period, its adoption was mostly limited to Roman Catholic nations, but by the 19th century it had become widely adopted for the sake of convenience in international trade. The last European country to adopt it was Greece, in 1923.

The calendar epoch used by the Gregorian calendar is inherited from the medieval convention established by Dionysius Exiguus and associated with the Julian calendar. The year number is variously given as AD (for Anno Domini) or CE (for Common Era or Christian Era).

Religious
The most important use of pre-modern calendars is keeping track of the liturgical year and the observation of religious feast days.

While the Gregorian calendar is itself historically motivated to the calculation of the Easter date, it is now in worldwide secular use as the de facto standard. Alongside the use of the Gregorian calendar for secular matters, there remain several calendars in use for religious purposes.

Western Christian liturgical calendars are based on the cycle of the Roman Rite of the Catholic Church and generally include the liturgical seasons of Advent, Christmas, Ordinary Time (Time after Epiphany), Lent, Easter, and Ordinary Time (Time after Pentecost). Some Christian calendars do not include Ordinary Time and every day falls into a denominated season.

The Eastern Orthodox Church employs the use of 2 liturgical calendars; the Julian calendar (often called the Old Calendar) and the Revised Julian Calendar (often called the New Calendar). The Revised Julian Calendar is nearly the same as the Gregorian calendar, with the addition that years divisible by 100 are not leap years, except that years with remainders of 200 or 600 when divided by 900 remain leap years, e.g. 2000 and 2400 as in the Gregorian calendar.

The Islamic calendar or Hijri calendar is a lunar calendar consisting of 12 lunar months in a year of 354 or 355 days. It is used to date events in most of the Muslim countries (concurrently with the Gregorian calendar) and used by Muslims everywhere to determine the proper day on which to celebrate Islamic holy days and festivals. Its epoch is the Hijra (corresponding to AD 622). With an annual drift of 11 or 12 days, the seasonal relation is repeated approximately every 33 Islamic years.

Various Hindu calendars remain in use in the Indian subcontinent, including the Nepali calendars, Bengali calendar, Malayalam calendar, Tamil calendar, Vikrama Samvat used in Northern India, and Shalivahana calendar in the Deccan states.

The Buddhist calendar and the traditional lunisolar calendars of Cambodia, Laos, Myanmar, Sri Lanka and Thailand are also based on an older version of the Hindu calendar.

Most of the Hindu calendars are inherited from a system first enunciated in Vedanga Jyotisha of Lagadha, standardized in the Sūrya Siddhānta and subsequently reformed by astronomers such as Āryabhaṭa (AD 499), Varāhamihira (6th century) and Bhāskara II (12th century).

The Hebrew calendar is used by Jews worldwide for religious and cultural affairs, also influences civil matters in Israel (such as national holidays) and can be used business dealings (such as for the dating of cheques).

Followers of the Baháʼí Faith use the Baháʼí calendar. The Baháʼí Calendar, also known as the Badi Calendar was first established by the Bab in the Kitab-i-Asma. The Baháʼí Calendar is also purely a solar calendar and comprises 19 months each having nineteen days.

National
The Chinese, Hebrew, Hindu, and Julian calendars are widely used for religious and social purposes.

The Iranian (Persian) calendar is used in Iran and some parts of Afghanistan. The Assyrian calendar is in use by the members of the Assyrian community in the Middle East (mainly Iraq, Syria, Turkey, and Iran) and the diaspora. The first year of the calendar is exactly 4750 years prior to the start of the Gregorian calendar. The Ethiopian calendar or Ethiopic calendar is the principal calendar used in Ethiopia and Eritrea, with the Oromo calendar also in use in some areas. In neighboring Somalia, the Somali calendar co-exists alongside the Gregorian and Islamic calendars. In Thailand, where the Thai solar calendar is used, the months and days have adopted the western standard, although the years are still based on the traditional Buddhist calendar.

Fiscal

A fiscal calendar generally means the accounting year of a government or a business. It is used for budgeting, keeping accounts, and taxation. It is a set of 12 months that may start at any date in a year. The US government's fiscal year starts on 1 October and ends on 30 September. The government of India's fiscal year starts on 1 April and ends on 31 March. Small traditional businesses in India start the fiscal year on Diwali festival and end the day before the next year's Diwali festival.

In accounting (and particularly accounting software), a fiscal calendar (such as a 4/4/5 calendar) fixes each month at a specific number of weeks to facilitate comparisons from month to month and year to year. January always has exactly 4 weeks (Sunday through Saturday), February has 4 weeks, March has 5 weeks, etc. Note that this calendar will normally need to add a 53rd week to every 5th or 6th year, which might be added to December or might not be, depending on how the organization uses those dates. There exists an international standard way to do this (the ISO week). The ISO week starts on a Monday and ends on a Sunday. Week 1 is always the week that contains 4 January in the Gregorian calendar.

Formats

The term calendar applies not only to a given scheme of timekeeping but also to a specific record or device displaying such a scheme, for example, an appointment book in the form of a pocket calendar (or personal organizer), desktop calendar, a wall calendar, etc.

In a paper calendar, one or two sheets can show a single day, a week, a month, or a year. If a sheet is for a single day, it easily shows the date and the weekday. If a sheet is for multiple days it shows a conversion table to convert from weekday to date and back. With a special pointing device, or by crossing out past days, it may indicate the current date and weekday. This is the most common usage of the word.

In the US Sunday is considered the first day of the week and so appears on the far left and Saturday the last day of the week appearing on the far right. In Britain, the weekend may appear at the end of the week so the first day is Monday and the last day is Sunday. The US calendar display is also used in Britain.

It is common to display the Gregorian calendar in separate monthly grids of seven columns (from Monday to Sunday, or Sunday to Saturday depending on which day is considered to start the week – this varies according to country) and five to six rows (or rarely, four rows when the month of February contains 28 days in common years beginning on the first day of the week), with the day of the month numbered in each cell, beginning with 1. The sixth row is sometimes eliminated by marking 23/30 and 24/31 together as necessary.

When working with weeks rather than months, a continuous format is sometimes more convenient, where no blank cells are inserted to ensure that the first day of a new month begins on a fresh row.

Software
Calendaring software provides users with an electronic version of a calendar, and may additionally provide an appointment book, address book, or contact list. Calendaring is a standard feature of many PDAs, EDAs, and smartphones. The software may be a local package designed for individual use (e.g., Lightning extension for Mozilla Thunderbird, Microsoft Outlook without Exchange Server, or Windows Calendar) or maybe a networked package that allows for the sharing of information between users (e.g., Mozilla Sunbird, Windows Live Calendar, Google Calendar, or Microsoft Outlook with Exchange Server).

title: Calendar