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Fundamental interaction

In physics, the fundamental interactions or fundamental forces are the interactions that do not appear to be reducible to more basic interactions. There are four fundamental interactions known to exist:
 * gravity
 * electromagnetism
 * weak interaction
 * strong interaction

The gravitational and electromagnetic interactions produce long-range forces whose effects can be seen directly in everyday life. The strong and weak interactions produce forces at minuscule, subatomic distances and govern nuclear interactions inside atoms.

Some scientists hypothesize that a fifth force might exist, but these hypotheses remain speculative. It is possible, however, that the fifth force is a combination of the prior four forces in the form of a scalar field; such as the Higgs field.

Each of the known fundamental interactions can be described mathematically as a field. The gravitational force is attributed to the curvature of spacetime, described by Einstein's general theory of relativity. The other three are discrete quantum fields, and their interactions are mediated by elementary particles described by the Standard Model of particle physics.

Within the Standard Model, the strong interaction is carried by a particle called the gluon and is responsible for quarks binding together to form hadrons, such as protons and neutrons. As a residual effect, it creates the nuclear force that binds the latter particles to form atomic nuclei. The weak interaction is carried by particles called W and Z bosons, and also acts on the nucleus of atoms, mediating radioactive decay. The electromagnetic force, carried by the photon, creates electric and magnetic fields, which are responsible for the attraction between orbital electrons and atomic nuclei which holds atoms together, as well as chemical bonding and electromagnetic waves, including visible light, and forms the basis for electrical technology. Although the electromagnetic force is far stronger than gravity, it tends to cancel itself out within large objects, so over large (astronomical) distances gravity tends to be the dominant force, and is responsible for holding together the large scale structures in the universe, such as planets, stars, and galaxies.

Many theoretical physicists believe these fundamental forces to be related and to become unified into a single force at very high energies on a minuscule scale, the Planck scale, but particle accelerators cannot produce the enormous energies required to experimentally probe this. Devising a common theoretical framework that would explain the relation between the forces in a single theory is perhaps the greatest goal of today's theoretical physicists. The weak and electromagnetic forces have already been unified with the electroweak theory of Sheldon Glashow, Abdus Salam, and Steven Weinberg, for which they received the 1979 Nobel Prize in physics. Some physicists seek to unite the electroweak and strong fields within what is called a Grand Unified Theory (GUT). An even bigger challenge is to find a way to quantize the gravitational field, resulting in a theory of quantum gravity (QG) which would unite gravity in a common theoretical framework with the other three forces. Some theories, notably string theory, seek both QG and GUT within one framework, unifying all four fundamental interactions along with mass generation within a theory of everything (ToE).

Classical theory
In his 1687 theory, Isaac Newton postulated space as an infinite and unalterable physical structure existing before, within, and around all objects while their states and relations unfold at a constant pace everywhere, thus absolute space and time. Inferring that all objects bearing mass approach at a constant rate, but collide by impact proportional to their masses, Newton inferred that matter exhibits an attractive force. His law of universal gravitation implied there to be instant interaction among all objects. As conventionally interpreted, Newton's theory of motion modelled a central force without a communicating medium. Thus Newton's theory violated the tradition, going back to Descartes, that there should be no action at a distance. Conversely, during the 1820s, when explaining magnetism, Michael Faraday inferred a field filling space and transmitting that force. Faraday conjectured that ultimately, all forces unified into one.

In 1873, James Clerk Maxwell unified electricity and magnetism as effects of an electromagnetic field whose third consequence was light, travelling at constant speed in vacuum. If his electromagnetic field theory held true in all inertial frames of reference, this would contradict Newton's theory of motion, which relied on Galilean relativity. If, instead, his field theory only applied to reference frames at rest relative to a mechanical luminiferous aether—presumed to fill all space whether within matter or in vacuum and to manifest the electromagnetic field—then it could be reconciled with Galilean relativity and Newton's laws. (However, such a "Maxwell aether" was later disproven; Newton's laws did, in fact, have to be replaced.)

Standard Model
The Standard Model of particle physics was developed throughout the latter half of the 20th century. In the Standard Model, the electromagnetic, strong, and weak interactions associate with elementary particles, whose behaviours are modelled in quantum mechanics (QM). For predictive success with QM's probabilistic outcomes, particle physics conventionally models QM events across a field set to special relativity, altogether relativistic quantum field theory (QFT). Force particles, called gauge bosons—force carriers or messenger particles of underlying fields—interact with matter particles, called fermions. Everyday matter is atoms, composed of three fermion types: up-quarks and down-quarks constituting, as well as electrons orbiting, the atom's nucleus. Atoms interact, form molecules, and manifest further properties through electromagnetic interactions among their electrons absorbing and emitting photons, the electromagnetic field's force carrier, which if unimpeded traverse potentially infinite distance. Electromagnetism's QFT is quantum electrodynamics (QED).

The force carriers of the weak interaction are the massive W and Z bosons. Electroweak theory (EWT) covers both electromagnetism and the weak interaction. At the high temperatures shortly after the Big Bang, the weak interaction, the electromagnetic interaction, and the Higgs boson were originally mixed components of a different set of ancient pre-symmetry-breaking fields. As the early universe cooled, these fields split into the long-range electromagnetic interaction, the short-range weak interaction, and the Higgs boson. In the Higgs mechanism, the Higgs field manifests Higgs bosons that interact with some quantum particles in a way that endows those particles with mass. The strong interaction, whose force carrier is the gluon, traversing minuscule distance among quarks, is modeled in quantum chromodynamics (QCD). EWT, QCD, and the Higgs mechanism comprise particle physics' Standard Model (SM). Predictions are usually made using calculational approximation methods, although such perturbation theory is inadequate to model some experimental observations (for instance bound states and solitons). Still, physicists widely accept the Standard Model as science's most experimentally confirmed theory.

Beyond the Standard Model, some theorists work to unite the electroweak and strong interactions within a Grand Unified Theory (GUT). Some attempts at GUTs hypothesize "shadow" particles, such that every known matter particle associates with an undiscovered force particle, and vice versa, altogether supersymmetry (SUSY). Other theorists seek to quantize the gravitational field by the modelling behaviour of its hypothetical force carrier, the graviton and achieve quantum gravity (QG). One approach to QG is loop quantum gravity (LQG). Still other theorists seek both QG and GUT within one framework, reducing all four fundamental interactions to a Theory of Everything (ToE). The most prevalent aim at a ToE is string theory, although to model matter particles, it added SUSY to force particles—and so, strictly speaking, became superstring theory. Multiple, seemingly disparate superstring theories were unified on a backbone, M-theory. Theories beyond the Standard Model remain highly speculative, lacking great experimental support.

Overview of the fundamental interactions

In the conceptual model of fundamental interactions, matter consists of fermions, which carry properties called charges and spin ±$1/2$ (intrinsic angular momentum ±$ħ/2$, where ħ is the reduced Planck constant). They attract or repel each other by exchanging bosons.

The interaction of any pair of fermions in perturbation theory can then be modelled thus:
 * Two fermions go in → interaction by boson exchange → two changed fermions go out.

The exchange of bosons always carries energy and momentum between the fermions, thereby changing their speed and direction. The exchange may also transport a charge between the fermions, changing the charges of the fermions in the process (e.g., turn them from one type of fermion to another). Since bosons carry one unit of angular momentum, the fermion's spin direction will flip from +$1/2$ to −$1/2$ (or vice versa) during such an exchange (in units of the reduced Planck constant). Since such interactions result in a change in momentum, they can give rise to classical Newtonian forces. In quantum mechanics, physicists often use the terms "force" and "interaction" interchangeably; for example, the weak interaction is sometimes referred to as the "weak force".

According to the present understanding, there are four fundamental interactions or forces: gravitation, electromagnetism, the weak interaction, and the strong interaction. Their magnitude and behaviour vary greatly, as described in the table below. Modern physics attempts to explain every observed physical phenomenon by these fundamental interactions. Moreover, reducing the number of different interaction types is seen as desirable. Two cases in point are the unification of:
 * Electric and magnetic force into electromagnetism;
 * The electromagnetic interaction and the weak interaction into the electroweak interaction; see below.

Both magnitude ("relative strength") and "range" of the associated potential, as given in the table, are meaningful only within a rather complex theoretical framework. The table below lists properties of a conceptual scheme that remains the subject of ongoing research.

The modern (perturbative) quantum mechanical view of the fundamental forces other than gravity is that particles of matter (fermions) do not directly interact with each other, but rather carry a charge, and exchange virtual particles (gauge bosons), which are the interaction carriers or force mediators. For example, photons mediate the interaction of electric charges, and gluons mediate the interaction of color charges. The full theory includes perturbations beyond simply fermions exchanging bosons; these additional perturbations can involve bosons that exchange fermions, as well as the creation or destruction of particles: see Feynman diagrams for examples.

Gravity
Gravitation is the weakest of the four interactions at the atomic scale, where electromagnetic interactions dominate.

Gravitation is the most important of the four fundamental forces for astronomical objects over astronomical distances for two reasons. First, gravitation has an infinite effective range, like electromagnetism but unlike the strong and weak interactions. Second, gravity always attracts and never repels; in contrast, astronomical bodies tend toward a near-neutral net electric charge, such that the attraction to one type of charge and the repulsion from the opposite charge mostly cancel each other out.

Even though electromagnetism is far stronger than gravitation, electrostatic attraction is not relevant for large celestial bodies, such as planets, stars, and galaxies, simply because such bodies contain equal numbers of protons and electrons and so have a net electric charge of zero. Nothing "cancels" gravity, since it is only attractive, unlike electric forces which can be attractive or repulsive. On the other hand, all objects having mass are subject to the gravitational force, which only attracts. Therefore, only gravitation matters on the large-scale structure of the universe.

The long range of gravitation makes it responsible for such large-scale phenomena as the structure of galaxies and black holes and, being only attractive, it retards the expansion of the universe. Gravitation also explains astronomical phenomena on more modest scales, such as planetary orbits, as well as everyday experience: objects fall; heavy objects act as if they were glued to the ground, and animals can only jump so high.

Gravitation was the first interaction to be described mathematically. In ancient times, Aristotle hypothesized that objects of different masses fall at different rates. During the Scientific Revolution, Galileo Galilei experimentally determined that this hypothesis was wrong under certain circumstances—neglecting the friction due to air resistance and buoyancy forces if an atmosphere is present (e.g. the case of a dropped air-filled balloon vs a water-filled balloon), all objects accelerate toward the Earth at the same rate. Isaac Newton's law of Universal Gravitation (1687) was a good approximation of the behaviour of gravitation. Present-day understanding of gravitation stems from Einstein's General Theory of Relativity of 1915, a more accurate (especially for cosmological masses and distances) description of gravitation in terms of the geometry of spacetime.

Merging general relativity and quantum mechanics (or quantum field theory) into a more general theory of quantum gravity is an area of active research. It is hypothesized that gravitation is mediated by a massless spin-2 particle called the graviton.

Although general relativity has been experimentally confirmed (at least for weak fields, i.e. not black holes) on all but the smallest scales, there are alternatives to general relativity. These theories must reduce to general relativity in some limit, and the focus of observational work is to establish limits on what deviations from general relativity are possible.

Proposed extra dimensions could explain why the gravity force is so weak.

Electroweak interaction
Electromagnetism and weak interaction appear to be very different at everyday low energies. They can be modeled using two different theories. However, above unification energy, on the order of 100 GeV, they would merge into a single electroweak force.

The electroweak theory is very important for modern cosmology, particularly on how the universe evolved. This is because shortly after the Big Bang, when the temperature was still above approximately 1015 K, the electromagnetic force and the weak force were still merged as a combined electroweak force.

For contributions to the unification of the weak and electromagnetic interaction between elementary particles, Abdus Salam, Sheldon Glashow and Steven Weinberg were awarded the Nobel Prize in Physics in 1979.

Electromagnetism
Electromagnetism is the force that acts between electrically charged particles. This phenomenon includes the electrostatic force acting between charged particles at rest, and the combined effect of electric and magnetic forces acting between charged particles moving relative to each other.

Electromagnetism has an infinite range like gravity, but is vastly stronger than it, and therefore describes several macroscopic phenomena of everyday experience such as friction, rainbows, lightning, and all human-made devices using electric current, such as television, lasers, and computers. Electromagnetism fundamentally determines all macroscopic, and many atomic-level, properties of the chemical elements, including all chemical bonding.

In a four kilogram (~1 gallon) jug of water, there is $$ 4000 \ \mbox{g}\,\rm{H}_2 \rm{O} \cdot \frac{1 \ \mbox{mol}\,\rm{H}_2 \rm{O}}{18 \ \mbox{g}\,H_2 O} \cdot \frac{10 \ \mbox{mol}\,e^{-}}{1 \ \mbox{mol}\,H_2 O} \cdot \frac{96,000 \ \mbox{C}\,}{1 \ \mbox{mol}\,e^{-}} = 2.1 \times 10^{8} C \ \, \ $$ of total electron charge. Thus, if we place two such jugs a meter apart, the electrons in one of the jugs repel those in the other jug with a force of $$ {1 \over 4\pi\varepsilon_0}\frac{(2.1 \times 10^{8} \mathrm{C})^2}{(1 m)^2} = 4.1 \times 10^{26} \mathrm{N}.$$

This force is many times larger than the weight of the planet Earth. The atomic nuclei in one jug also repel those in the other with the same force. However, these repulsive forces are canceled by the attraction of the electrons in jug A with the nuclei in jug B and the attraction of the nuclei in jug A with the electrons in jug B, resulting in no net force. Electromagnetic forces are tremendously stronger than gravity but cancel out so that for large bodies gravity dominates.

Electrical and magnetic phenomena have been observed since ancient times, but it was only in the 19th century James Clerk Maxwell discovered that electricity and magnetism are two aspects of the same fundamental interaction. By 1864, Maxwell's equations had rigorously quantified this unified interaction. Maxwell's theory, restated using vector calculus, is the classical theory of electromagnetism, suitable for most technological purposes.

The constant speed of light in vacuum (customarily denoted with a lowercase letter $c$) can be derived from Maxwell's equations, which are consistent with the theory of special relativity. Albert Einstein's 1905 theory of special relativity, however, which follows from the observation that the speed of light is constant no matter how fast the observer is moving, showed that the theoretical result implied by Maxwell's equations has profound implications far beyond electromagnetism on the very nature of time and space.

In another work that departed from classical electro-magnetism, Einstein also explained the photoelectric effect by utilizing Max Planck's discovery that light was transmitted in 'quanta' of specific energy content based on the frequency, which we now call photons. Starting around 1927, Paul Dirac combined quantum mechanics with the relativistic theory of electromagnetism. Further work in the 1940s, by Richard Feynman, Freeman Dyson, Julian Schwinger, and Sin-Itiro Tomonaga, completed this theory, which is now called quantum electrodynamics, the revised theory of electromagnetism. Quantum electrodynamics and quantum mechanics provide a theoretical basis for electromagnetic behavior such as quantum tunneling, in which a certain percentage of electrically charged particles move in ways that would be impossible under the classical electromagnetic theory, that is necessary for everyday electronic devices such as transistors to function.

Weak interaction
The weak interaction or weak nuclear force is responsible for some nuclear phenomena such as beta decay. Electromagnetism and the weak force are now understood to be two aspects of a unified electroweak interaction — this discovery was the first step toward the unified theory known as the Standard Model. In the theory of the electroweak interaction, the carriers of the weak force are the massive gauge bosons called the W and Z bosons. The weak interaction is the only known interaction that does not conserve parity; it is left–right asymmetric. The weak interaction even violates CP symmetry but does conserve CPT.

Strong interaction
The strong interaction, or strong nuclear force, is the most complicated interaction, mainly because of the way it varies with distance. The nuclear force is powerfully attractive between nucleons at distances of about 1 femtometre (fm, or 10−15 metres), but it rapidly decreases to insignificance at distances beyond about 2.5 fm. At distances less than 0.7 fm, the nuclear force becomes repulsive. This repulsive component is responsible for the physical size of nuclei, since the nucleons can come no closer than the force allows.

After the nucleus was discovered in 1908, it was clear that a new force, today known as the nuclear force, was needed to overcome the electrostatic repulsion, a manifestation of electromagnetism, of the positively charged protons. Otherwise, the nucleus could not exist. Moreover, the force had to be strong enough to squeeze the protons into a volume whose diameter is about 10−15 m, much smaller than that of the entire atom. From the short range of this force, Hideki Yukawa predicted that it was associated with a massive force particle, whose mass is approximately 100 MeV.

The 1947 discovery of the pion ushered in the modern era of particle physics. Hundreds of hadrons were discovered from the 1940s to 1960s, and an extremely complicated theory of hadrons as strongly interacting particles was developed. Most notably:
 * The pions were understood to be oscillations of vacuum condensates;
 * Jun John Sakurai proposed the rho and omega vector bosons to be force carrying particles for approximate symmetries of isospin and hypercharge;
 * Geoffrey Chew, Edward K. Burdett and Steven Frautschi grouped the heavier hadrons into families that could be understood as vibrational and rotational excitations of strings.

While each of these approaches offered insights, no approach led directly to a fundamental theory.

Murray Gell-Mann along with George Zweig first proposed fractionally charged quarks in 1961. Throughout the 1960s, different authors considered theories similar to the modern fundamental theory of quantum chromodynamics (QCD) as simple models for the interactions of quarks. The first to hypothesize the gluons of QCD were Moo-Young Han and Yoichiro Nambu, who introduced the quark color charge. Han and Nambu hypothesized that it might be associated with a force-carrying field. At that time, however, it was difficult to see how such a model could permanently confine quarks. Han and Nambu also assigned each quark color an integer electrical charge, so that the quarks were fractionally charged only on average, and they did not expect the quarks in their model to be permanently confined.

In 1971, Murray Gell-Mann and Harald Fritzsch proposed that the Han/Nambu color gauge field was the correct theory of the short-distance interactions of fractionally charged quarks. A little later, David Gross, Frank Wilczek, and David Politzer discovered that this theory had the property of asymptotic freedom, allowing them to make contact with experimental evidence. They concluded that QCD was the complete theory of the strong interactions, correct at all distance scales. The discovery of asymptotic freedom led most physicists to accept QCD since it became clear that even the long-distance properties of the strong interactions could be consistent with experiment if the quarks are permanently confined: the strong force increases indefinitely with distance, trapping quarks inside the hadrons.

Assuming that quarks are confined, Mikhail Shifman, Arkady Vainshtein and Valentine Zakharov were able to compute the properties of many low-lying hadrons directly from QCD, with only a few extra parameters to describe the vacuum. In 1980, Kenneth G. Wilson published computer calculations based on the first principles of QCD, establishing, to a level of confidence tantamount to certainty, that QCD will confine quarks. Since then, QCD has been the established theory of strong interactions.

QCD is a theory of fractionally charged quarks interacting by means of 8 bosonic particles called gluons. The gluons also interact with each other, not just with the quarks, and at long distances the lines of force collimate into strings, loosely modeled by a linear potential, a constant attractive force. In this way, the mathematical theory of QCD not only explains how quarks interact over short distances but also the string-like behavior, discovered by Chew and Frautschi, which they manifest over longer distances.

Higgs interaction
Conventionally, the Higgs interaction is not counted among the four fundamental forces.

Nonetheless, although not a gauge interaction nor generated by any diffeomorphism symmetry, the Higgs field's cubic Yukawa coupling produces a weakly attractive fifth interaction. After spontaneous symmetry breaking via the Higgs mechanism, Yukawa terms remain of the form
 * $$\frac{\lambda_i}{\sqrt 2} \bar{\psi} \phi' \psi = \frac{m_i}{\nu} \bar{\psi} \phi' \psi$$,

with Yukawa coupling $$\lambda_i$$, particle mass $$m_i$$ (in eV), and Higgs vacuum expectation value $246.22 GeV$. Hence coupled particles can exchange a virtual Higgs boson, yielding classical potentials of the form
 * $$V(r) = - \frac{m_i m_j}{m_{\rm H}^2} \frac{1}{4\pi r} e^{-m_{\rm H}\, c\, r/\hbar}$$,

with Higgs mass $125.18 GeV$. Because the reduced Compton wavelength of the Higgs boson is so small ($0 m$, comparable to the W and Z bosons), this potential has an effective range of a few attometers. Between two electrons, it begins roughly 1011 times weaker than the weak interaction, and grows exponentially weaker at non-zero distances.

Beyond the Standard Model
Numerous theoretical efforts have been made to systematize the existing four fundamental interactions on the model of electroweak unification.

Grand Unified Theories (GUTs) are proposals to show that the three fundamental interactions described by the Standard Model are all different manifestations of a single interaction with symmetries that break down and create separate interactions below some extremely high level of energy. GUTs are also expected to predict some of the relationships between constants of nature that the Standard Model treats as unrelated, as well as predicting gauge coupling unification for the relative strengths of the electromagnetic, weak, and strong forces (this was, for example, verified at the Large Electron–Positron Collider in 1991 for supersymmetric theories).

Theories of everything, which integrate GUTs with a quantum gravity theory face a greater barrier, because no quantum gravity theories, which include string theory, loop quantum gravity, and twistor theory, have secured wide acceptance. Some theories look for a graviton to complete the Standard Model list of force-carrying particles, while others, like loop quantum gravity, emphasize the possibility that time-space itself may have a quantum aspect to it.

Some theories beyond the Standard Model include a hypothetical fifth force, and the search for such a force is an ongoing line of experimental physics research. In supersymmetric theories, some particles acquire their masses only through supersymmetry breaking effects and these particles, known as moduli, can mediate new forces. Another reason to look for new forces is the discovery that the expansion of the universe is accelerating (also known as dark energy), giving rise to a need to explain a nonzero cosmological constant, and possibly to other modifications of general relativity. Fifth forces have also been suggested to explain phenomena such as CP violations, dark matter, and dark flow.

title: Fundamental interaction


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Floppy disk

A floppy disk or floppy diskette (casually referred to as a floppy, a diskette, or a disk) is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a fabric that removes dust particles from the spinning disk. The three most popular (and commercially available) floppy disks are the 8-inch, 5¼-inch, and 3½-inch floppy disks. Floppy disks store digital data which can be read and written when the disk is inserted into a floppy disk drive (FDD) connected to or inside a computer or other device.

The first floppy disks, invented and made by IBM in 1971, had a disk diameter of 8 in. Subsequently, the 5¼-inch (133.35 mm) and then the 3½-inch (88.9 mm) became a ubiquitous form of data storage and transfer into the first years of the 21st century. 3½-inch floppy disks can still be used with an external USB floppy disk drive. USB drives for 5¼-inch, 8-inch, and other-size floppy disks are rare to non-existent. Some individuals and organizations continue to use older equipment to read or transfer data from floppy disks.

Floppy disks were so common in late 20th-century culture that many electronic and software programs continue to use save icons that look like floppy disks well into the 21st century, as a form of skeuomorphic design. While floppy disk drives still have some limited uses, especially with legacy industrial computer equipment, they have been superseded by data storage methods with much greater data storage capacity and data transfer speed, such as USB flash drives, memory cards, optical discs, and storage available through local computer networks and cloud storage.

History

The first commercial floppy disks, developed in the late 1960s, were 8 in in diameter; they became commercially available in 1971 as a component of IBM products and both drives and disks were then sold separately starting in 1972 by Memorex and others. These disks and associated drives were produced and improved upon by IBM and other companies such as Memorex, Shugart Associates, and Burroughs Corporation. The term "floppy disk" appeared in print as early as 1970, and although IBM announced its first media as the Type 1 Diskette in 1973, the industry continued to use the terms "floppy disk" or "floppy".

In 1976, Shugart Associates introduced the 5¼-inch FDD. By 1978, there were more than ten manufacturers producing such FDDs. There were competing floppy disk formats, with hard- and soft-sector versions and encoding schemes such as differential Manchester encoding (DM), modified frequency modulation (MFM), M2FM and group coded recording (GCR). The 5¼-inch format displaced the 8-inch one for most uses, and the hard-sectored disk format disappeared. The most common capacity of the 5¼-inch format in DOS-based PCs was 360 KB (368,640 bytes) for the Double-Sided Double-Density (DSDD) format using MFM encoding.

In 1984, IBM introduced with its PC/AT the 1.2 MB (1,228,800 bytes) dual-sided 5¼-inch floppy disk, but it never became very popular. IBM started using the 720 KB double density 3½-inch microfloppy disk on its Convertible laptop computer in 1986 and the 1.44 MB high-density version with the IBM Personal System/2 (PS/2) line in 1987. These disk drives could be added to older PC models. In 1988, Y-E Data introduced a drive for 2.88 MB Double-Sided Extended-Density (DSED) diskettes which was used by IBM in its top-of-the-line PS/2 and some RS/6000 models and in the second-generation NeXTcube and NeXTstation; however, this format had limited market success due to lack of standards and movement to 1.44 MB drives.

Throughout the early 1980s, limits of the 5¼-inch format became clear. Originally designed to be more practical than the 8-inch format, it was becoming considered too large; as the quality of recording media grew, data could be stored in a smaller area. Several solutions were developed, with drives at 2-, 2½-, 3-, 3¼-, 3½- and 4-inches (and Sony's 90 x disk) offered by various companies. They all had several advantages over the old format, including a rigid case with a sliding metal (or later, sometimes plastic) shutter over the head slot, which helped protect the delicate magnetic medium from dust and damage, and a sliding write protection tab, which was far more convenient than the adhesive tabs used with earlier disks. The large market share of the well-established 5¼-inch format made it difficult for these diverse mutually-incompatible new formats to gain significant market share. A variant on the Sony design, introduced in 1983 by many manufacturers, was then rapidly adopted. By 1988, the 3½-inch was outselling the 5¼-inch.

Generally, the term floppy disk persisted, even though later style floppy disks have a rigid case around an internal floppy disk.

By the end of the 1980s, 5¼-inch disks had been superseded by 3½-inch disks. During this time, PCs frequently came equipped with drives of both sizes. By the mid-1990s, 5¼-inch drives had virtually disappeared, as the 3½-inch disk became the predominant floppy disk. The advantages of the 3½-inch disk were its higher capacity, its smaller physical size, and its rigid case which provided better protection from dirt and other environmental risks.

Prevalence

Floppy disks became commonplace during the 1980s and 1990s in their use with personal computers to distribute software, transfer data, and create backups. Before hard disks became affordable to the general population, floppy disks were often used to store a computer's operating system (OS). Most home computers from that time have an elementary OS and BASIC stored in read-only memory (ROM), with the option of loading a more advanced OS from a floppy disk.

By the early 1990s, the increasing software size meant large packages like Windows or Adobe Photoshop required a dozen disks or more. In 1996, there were an estimated five billion standard floppy disks in use.

An attempt to enhance the existing 3½-inch designs was the SuperDisk in the late 1990s, using very narrow data tracks and a high precision head guidance mechanism with a capacity of 120 MB and backward-compatibility with standard 3½-inch floppies; a format war briefly occurred between SuperDisk and other high-density floppy-disk products, although ultimately recordable CDs/DVDs, solid-state flash storage, and eventually cloud-based online storage would render all these removable disk formats obsolete. External USB-based floppy disk drives are still available, and many modern systems provide firmware support for booting from such drives.

Gradual transition to other formats

In the mid-1990s, mechanically incompatible higher-density floppy disks were introduced, like the Iomega Zip disk. Adoption was limited by the competition between proprietary formats and the need to buy expensive drives for computers where the disks would be used. In some cases, failure in market penetration was exacerbated by the release of higher-capacity versions of the drive and media being not backward-compatible with the original drives, dividing the users between new and old adopters. Consumers were wary of making costly investments into unproven and rapidly changing technologies, so none of the technologies became the established standard.

Apple introduced the iMac G3 in 1998 with a CD-ROM drive but no floppy drive; this made USB-connected floppy drives popular accessories, as the iMac came without any writable removable media device.

Recordable CDs were touted as an alternative, because of the greater capacity, compatibility with existing CD-ROM drives, and—with the advent of re-writeable CDs and packet writing—a similar reusability as floppy disks. However, CD-R/RWs remained mostly an archival medium, not a medium for exchanging data or editing files on the medium itself, because there was no common standard for packet writing which allowed for small updates. Other formats, such as magneto-optical discs, had the flexibility of floppy disks combined with greater capacity, but remained niche due to costs. High-capacity backward compatible floppy technologies became popular for a while and were sold as an option or even included in standard PCs, but in the long run, their use was limited to professionals and enthusiasts.

Flash-based USB-thumb drives finally were a practical and popular replacement, that supported traditional file systems and all common usage scenarios of floppy disks. As opposed to other solutions, no new drive type or special software was required that impeded adoption, since all that was necessary was an already common USB port.

Usage in the 21st century

By 2002, most manufacturers still provided floppy disk drives as standard equipment to meet user demand for file-transfer and an emergency boot device, as well as for the general secure feeling of having the familiar device. By this time, the retail cost of a floppy drive had fallen to around $20, so there was little financial incentive to omit the device from a system. Subsequently, enabled by the widespread support for USB flash drives and BIOS boot, manufacturers and retailers progressively reduced the availability of floppy disk drives as standard equipment. In February 2003, Dell, one of the leading personal computer vendors, announced that floppy drives would no longer be pre-installed on Dell Dimension home computers, although they were still available as a selectable option and purchasable as an aftermarket OEM add-on. By January 2007, only 2% of computers sold in stores contained built-in floppy disk drives.

Floppy disks are used for emergency boots in aging systems lacking support for other bootable media and for BIOS updates, since most BIOS and firmware programs can still be executed from bootable floppy disks. If BIOS updates fail or become corrupt, floppy drives can sometimes be used to perform a recovery. The music and theatre industries still use equipment requiring standard floppy disks (e.g. synthesizers, samplers, drum machines, sequencers, and lighting consoles). Industrial automation equipment such as programmable machinery and industrial robots may not have a USB interface; data and programs are then loaded from disks, damageable in industrial environments. This equipment may not be replaced due to cost or requirement for continuous availability; existing software emulation and virtualization do not solve this problem because a customized operating system is used that has no drivers for USB devices. Hardware floppy disk emulators can be made to interface floppy-disk controllers to a USB port that can be used for flash drives.

In May 2016, the United States Government Accountability Office released a report that covered the need to upgrade or replace legacy computer systems within federal agencies. According to this document, old IBM Series/1 minicomputers running on 8-inch floppy disks are still used to coordinate "the operational functions of the United States' nuclear forces". The government planned to update some of the technology by the end of the 2017 fiscal year. Use in Japan's government ended in 2024.

Windows 10 and Windows 11 no longer come with drivers for floppy disk drives (both internal and external). However, they will still support them with a separate device driver provided by Microsoft.

The British Airways Boeing 747-400 fleet, up to its retirement in 2020, used 3½-inch floppy disks to load avionics software.

Sony, who had been in the floppy disk business since 1983, ended domestic sales of all six 3½-inch floppy disk models as of March 2011. This has been viewed by some as the end of the floppy disk. While production of new floppy disk media has ceased, sales and uses of this media from inventories is expected to continue until at least 2026.

Legacy

For more than two decades, the floppy disk was the primary external writable storage device used. Most computing environments before the 1990s were non-networked, and floppy disks were the primary means to transfer data between computers, a method known informally as sneakernet. Unlike hard disks, floppy disks are handled and seen; even a novice user can identify a floppy disk. Because of these factors, a picture of a 3½-inch floppy disk became an interface metaphor for saving data. The floppy disk symbol is still used by software on user-interface elements related to saving files (such as LibreOffice) even though physical floppy disks are largely obsolete.

8-inch and 5¼-inch disks
The 8-inch and 5¼-inch floppy disks contain a magnetically coated round plastic medium with a large circular hole in the center for a drive's spindle. The medium is contained in a square plastic cover that has a small oblong opening in both sides to allow the drive's heads to read and write data and a large hole in the center to allow the magnetic medium to spin by rotating it from its middle hole.

Inside the cover are two layers of fabric with the magnetic medium sandwiched in the middle. The fabric is designed to reduce friction between the medium and the outer cover, and catch particles of debris abraded off the disk to keep them from accumulating on the heads. The cover is usually a one-part sheet, double-folded with flaps glued or spot-welded together.

A small notch on the side of the disk identifies whether it is writable, as detected by a mechanical switch or photoelectric sensor. In the 8-inch disk, the notch being covered or not present enables writing, while in the 5¼-inch disk, the notch being present and uncovered enables writing. Tape may be used over the notch to change the mode of the disk. Punch devices were sold to convert read-only 5¼" disks to writable ones, and also to enable writing on the unused side of single-sided disks for computers with single-sided drives. The latter worked because single- and double-sided disks typically contained essentially identical actual magnetic media, for manufacturing efficiency. Disks whose obverse and reverse sides were thus used separately in single-sided drives were known as flippy disks. Disk notching 5¼" floppies for PCs was generally only required where users wanted to overwrite original 5¼" disks of store-bought software, which somewhat commonly shipped with no notch present.

Another LED/photo-transistor pair located near the center of the disk detects the index hole once per rotation in the magnetic disk. Detection occurs whenever the drive's sensor, the holes in the correctly inserted floppy's plastic envelope and a single hole in the rotating floppy disk medium line up. This mechanism is used to detect the angular start of each track, and whether or not the disk is rotating at the correct speed. Early 8‑inch and 5¼‑inch disks also had holes for each sector in the enclosed magnetic medium, in addition to the index hole, with the same radial distance from the center, for alignment with the same envelope hole. These were termed hard sectored disks. Later soft-sectored disks have only one index hole in the medium, and sector position is determined by the disk controller or low-level software from patterns marking the start of a sector. Generally, the same drives are used to read and write both types of disks, with only the disks and controllers differing. Some operating systems using soft sectors, such as Apple DOS, do not use the index hole, and the drives designed for such systems often lack the corresponding sensor; this was mainly a hardware cost-saving measure.

3½-inch disk

The core of the 3½-inch disk is the same as the other two disks, but the front has only a label and a small opening for reading and writing data, protected by the shutter—a spring-loaded metal or plastic cover, pushed to the side on entry into the drive. Rather than having a hole in the center, it has a metal hub which mates to the spindle of the drive. Typical 3½-inch disk magnetic coating materials are:

 * DD: 2 μm magnetic iron oxide
 * HD: 1.2 μm cobalt-doped iron oxide
 * ED: 3 μm barium ferrite

Two holes at the bottom left and right indicate whether the disk is write-protected and whether it is high-density; these holes are spaced as far apart as the holes in punched A4 paper, allowing write-protected high-density floppy disks to be clipped into international standard (ISO 838) ring binders. The dimensions of the disk shell are not quite square: its width is slightly less than its depth, so that it is impossible to insert the disk into a drive slot sideways (i.e. rotated 90 degrees from the correct shutter-first orientation). A diagonal notch at top right ensures that the disk is inserted into the drive in the correct orientation—not upside down or label-end first—and an arrow at top left indicates direction of insertion. The drive usually has a button that, when pressed, ejects the disk with varying degrees of force, the discrepancy due to the ejection force provided by the spring of the shutter. In IBM PC compatibles, Commodores, Apple II/IIIs, and other non-Apple-Macintosh machines with standard floppy disk drives, a disk may be ejected manually at any time. The drive has a disk-change switch that detects when a disk is ejected or inserted. Failure of this mechanical switch is a common source of disk corruption if a disk is changed and the drive (and hence the operating system) fails to notice.

One of the chief usability problems of the floppy disk is its vulnerability; even inside a closed plastic housing, the disk medium is highly sensitive to dust, condensation and temperature extremes. As with all magnetic storage, it is vulnerable to magnetic fields. Blank disks have been distributed with an extensive set of warnings, cautioning the user not to expose it to dangerous conditions. Rough treatment or removing the disk from the drive while the magnetic media is still spinning is likely to cause damage to the disk, drive head, or stored data. On the other hand, the 3½‑inch floppy disk has been lauded for its mechanical usability by human–computer interaction expert Donald Norman:

"A simple example of a good design is the 3½-inch magnetic diskette for computers, a small circle of floppy magnetic material encased in hard plastic. Earlier types of floppy disks did not have this plastic case, which protects the magnetic material from abuse and damage. A sliding metal cover protects the delicate magnetic surface when the diskette is not in use and automatically opens when the diskette is inserted into the computer. The diskette has a square shape: there are apparently eight possible ways to insert it into the machine, only one of which is correct. What happens if I do it wrong? I try inserting the disk sideways. Ah, the designer thought of that. A little study shows that the case really isn't square: it's rectangular, so you can't insert a longer side. I try backward. The diskette goes in only part of the way. Small protrusions, indentations, and cutouts prevent the diskette from being inserted backward or upside down: of the eight ways one might try to insert the diskette, only one is correct, and only that one will fit. An excellent design."

Operation

A spindle motor in the drive rotates the magnetic medium at a certain speed, while a stepper motor-operated mechanism moves the magnetic read/write heads radially along the surface of the disk. Both read and write operations require the media to be rotating and the head to contact the disk media, an action originally accomplished by a disk-load solenoid. Later drives held the heads out of contact until a front-panel lever was rotated (5¼-inch) or disk insertion was complete (3½-inch). To write data, current is sent through a coil in the head as the media rotates. The head's magnetic field aligns the magnetization of the particles directly below the head on the media. When the current is reversed the magnetization aligns in the opposite direction, encoding one bit of data. To read data, the magnetization of the particles in the media induce a tiny voltage in the head coil as they pass under it. This small signal is amplified and sent to the floppy disk controller, which converts the streams of pulses from the media into data, checks it for errors, and sends it to the host computer system.

Formatting
A blank unformatted diskette has a coating of magnetic oxide with no magnetic order to the particles. During formatting, the magnetizations of the particles are aligned forming tracks, each broken up into sectors, enabling the controller to properly read and write data. The tracks are concentric rings around the center, with spaces between tracks where no data is written; gaps with padding bytes are provided between the sectors and at the end of the track to allow for slight speed variations in the disk drive, and to permit better interoperability with disk drives connected to other similar systems.

Each sector of data has a header that identifies the sector location on the disk. A cyclic redundancy check (CRC) is written into the sector headers and at the end of the user data so that the disk controller can detect potential errors.

Some errors are soft and can be resolved by automatically re-trying the read operation; other errors are permanent and the disk controller will signal a failure to the operating system if multiple attempts to read the data still fail.

Insertion and ejection
After a disk is inserted, a catch or lever at the front of the drive is manually lowered to prevent the disk from accidentally emerging, engage the spindle clamping hub, and in two-sided drives, engage the second read/write head with the media.

In some 5¼-inch drives, insertion of the disk compresses and locks an ejection spring which partially ejects the disk upon opening the catch or lever. This enables a smaller concave area for the thumb and fingers to grasp the disk during removal.

Newer 5¼-inch drives and all 3½-inch drives automatically engage the spindle and heads when a disk is inserted, doing the opposite with the press of the eject button.

On Apple Macintosh computers with built-in 3½-inch disk drives, the ejection button is replaced by software controlling an ejection motor which only does so when the operating system no longer needs to access the drive. The user could drag the image of the floppy drive to the trash can on the desktop to eject the disk. In the case of a power failure or drive malfunction, a loaded disk can be removed manually by inserting a straightened paper clip into a small hole at the drive's front panel, just as one would do with a CD-ROM drive in a similar situation. The X68000 has soft-eject 5¼-inch drives. Some late-generation IBM PS/2 machines had soft-eject 3½-inch disk drives as well for which some issues of DOS (i.e. PC DOS 5.02 and higher) offered an EJECT command.

Finding track zero
Before a disk can be accessed, the drive needs to synchronize its head position with the disk tracks. In some drives, this is accomplished with a Track Zero Sensor, while for others it involves the drive head striking an immobile reference surface.

In either case, the head is moved so that it is approaching track zero position of the disk. When a drive with the sensor has reached track zero, the head stops moving immediately and is correctly aligned. For a drive without the sensor, the mechanism attempts to move the head the maximum possible number of positions needed to reach track zero, knowing that once this motion is complete, the head will be positioned over track zero.

Some drive mechanisms such as the Apple II 5¼-inch drive without a track zero sensor, produce characteristic mechanical noises when trying to move the heads past the reference surface. This physical striking is responsible for the 5¼-inch drive clicking during the boot of an Apple II, and the loud rattles of its DOS and ProDOS when disk errors occurred and track zero synchronization was attempted.

Finding sectors
All 8-inch and some 5¼-inch drives used a mechanical method to locate sectors, known as either hard sectors or soft sectors, and is the purpose of the small hole in the jacket, off to the side of the spindle hole. A light beam sensor detects when a punched hole in the disk is visible through the hole in the jacket.

For a soft-sectored disk, there is only a single hole, which is used to locate the first sector of each track. Clock timing is then used to find the other sectors behind it, which requires precise speed regulation of the drive motor.

For a hard-sectored disk, there are many holes, one for each sector row, plus an additional hole in a half-sector position, that is used to indicate sector zero.

The Apple II computer system is notable in that it did not have an index hole sensor and ignored the presence of hard or soft sectoring. Instead, it used special repeating data synchronization patterns written to the disk between each sector, to assist the computer in finding and synchronizing with the data in each track.

The later 3½-inch drives of the mid-1980s did not use sector index holes, but instead also used synchronization patterns.

Most 3½-inch drives used a constant speed drive motor and contain the same number of sectors across all tracks. This is sometimes referred to as Constant Angular Velocity (CAV). In order to fit more data onto a disk, some 3½-inch drives (notably the Macintosh External 400K and 800K drives) instead use Constant Linear Velocity (CLV), which uses a variable speed drive motor that spins more slowly as the head moves away from the center of the disk, maintaining the same speed of the head(s) relative to the surface(s) of the disk. This allows more sectors to be written to the longer middle and outer tracks as the track length increases.

Sizes
While the original IBM 8-inch disk was actually so defined, the other sizes are defined in the metric system, their usual names being but rough approximations.

Different sizes of floppy disks are mechanically incompatible, and disks can fit only one size of drive. Drive assemblies with both 3½-inch and 5¼-inch slots were available during the transition period between the sizes, but they contained two separate drive mechanisms. In addition, there are many subtle, usually software-driven incompatibilities between the two. 5¼-inch disks formatted for use with Apple II computers would be unreadable and treated as unformatted on a Commodore. As computer platforms began to form, attempts were made at interchangeability. For example, the "SuperDrive" included from the Macintosh SE to the Power Macintosh G3 could read, write and format IBM PC format 3½-inch disks, but few IBM-compatible computers had drives that did the reverse. 8-inch, 5¼-inch and 3½-inch drives were manufactured in a variety of sizes, most to fit standardized drive bays. Alongside the common disk sizes were non-classical sizes for specialized systems.

8-inch floppy disk
Floppy disks of the first standard are 8 inches in diameter, protected by a flexible plastic jacket. It was a read-only device used by IBM as a way of loading microcode. Read/write floppy disks and their drives became available in 1972, but it was IBM's 1973 introduction of the 3740 data entry system that began the establishment of floppy disks, called by IBM the Diskette 1, as an industry standard for information interchange. Formatted diskette for this system store 242,944 bytes. Early microcomputers used for engineering, business, or word processing often used one or more 8-inch disk drives for removable storage; the CP/M operating system was developed for microcomputers with 8-inch drives.

The family of 8-inch disks and drives increased over time and later versions could store up to 1.2 MB; many microcomputer applications did not need that much capacity on one disk, so a smaller size disk with lower-cost media and drives was feasible. The 5¼-inch drive succeeded the 8-inch size in many applications, and developed to about the same storage capacity as the original 8-inch size, using higher-density media and recording techniques.

5¼-inch floppy disk
The head gap of an 80‑track high-density (1.2 MB in the MFM format) 5¼‑inch drive (a.k.a. Mini diskette, Mini disk, or Minifloppy) is smaller than that of a 40‑track double-density (360 KB if double-sided) drive but can also format, read and write 40‑track disks provided the controller supports double stepping or has a switch to do so. 5¼-inch 80-track drives were also called hyper drives. A blank 40‑track disk formatted and written on an 80‑track drive can be taken to its native drive without problems, and a disk formatted on a 40‑track drive can be used on an 80‑track drive. Disks written on a 40‑track drive and then updated on an 80 track drive become unreadable on any 40‑track drives due to track width incompatibility.

Single-sided disks were coated on both sides, despite the availability of more expensive double sided disks. The reason usually given for the higher price was that double sided disks were certified error-free on both sides of the media. Double-sided disks could be used in some drives for single-sided disks, as long as an index signal was not needed. This was done one side at a time, by turning them over (flippy disks); more expensive dual-head drives which could read both sides without turning over were later produced, and eventually became used universally.

3½-inch floppy disk
[[File:Floppy disk internal diagram.svg|thumb|upright|Internal parts of a 3½-inch floppy disk. 1. A hole that indicates a high-capacity disk.

2. The hub that engages with the drive motor.

3. A shutter that protects the surface when removed from the drive.

4. The plastic housing.

5. A polyester sheet reducing friction against the disk media as it rotates within the housing.

6. The magnetic coated plastic disk.

7. A schematic representation of one sector of data on the disk; the tracks and sectors are not visible on actual disks.

8. The write protection tab (unlabeled) in upper left.]]

In the early 1980s, many manufacturers introduced smaller floppy drives and media in various formats. A consortium of 21 companies eventually settled on a 3½-inch design known as the Micro diskette, Micro disk, or Micro floppy, similar to a Sony design but improved to support both single-sided and double-sided media, with formatted capacities generally of 360 KB and 720 KB respectively. Single-sided drives of the consortium design first shipped in 1983, and double-sided in 1984. The double-sided, high-density 1.44 MB (actually 1440 KiB = 1.41 MiB) disk drive, which would become the most popular, first shipped in 1986. The first Macintosh computers used single-sided 3½-inch floppy disks, but with 400 KB formatted capacity. These were followed in 1986 by double-sided 800 KB floppies. The higher capacity was achieved at the same recording density by varying the disk-rotation speed with head position so that the linear speed of the disk was closer to constant. Later Macs could also read and write 1.44 MB HD disks in PC format with fixed rotation speed. Higher capacities were similarly achieved by Acorn's RISC OS (800 KB for DD, 1,600 KB for HD) and AmigaOS (880 KB for DD, 1,760 KB for HD).

All 3½-inch disks have a rectangular hole in one corner which, if obstructed, write-enables the disk. A sliding detented piece can be moved to block or reveal the part of the rectangular hole that is sensed by the drive. The HD 1.44 MB disks have a second, unobstructed hole in the opposite corner that identifies them as being of that capacity.

In IBM-compatible PCs, the three densities of 3½-inch floppy disks are backwards-compatible; higher-density drives can read, write and format lower-density media. It is also possible to format a disk at a lower density than that for which it was intended, but only if the disk is first thoroughly demagnetized with a bulk eraser, as the high-density format is magnetically stronger and will prevent the disk from working in lower-density modes.

Writing at different densities than those at which disks were intended, sometimes by altering or drilling holes, was possible but not supported by manufacturers. A hole on one side of a 3½-inch disk can be altered as to make some disk drives and operating systems treat the disk as one of higher or lower density, for bidirectional compatibility or economical reasons. Some computers, such as the PS/2 and Acorn Archimedes, ignored these holes altogether.

Other sizes
Other smaller floppy sizes were proposed, especially for portable or pocket-sized devices that needed a smaller storage device. None of these sizes achieved much market success.
 * 3¼-inch floppies otherwise similar to 5¼-inch floppies were proposed by Tabor and Dysan.
 * Three-inch disks similar in construction to 3½-inch were manufactured and used for a time, particularly by Amstrad computers and word processors.
 * A two-inch nominal size known as the Video Floppy was introduced by Sony for use with its Mavica still video camera.
 * An incompatible two-inch floppy produced by Fujifilm called the LT-1 was used in the Zenith Minisport portable computer.

Sizes, performance and capacity
Floppy disk size is often referred to in inches, even in countries using metric and though the size is defined in metric. The ANSI specification of 3½-inch disks is entitled in part "90 mm (3.5-inch)" though 90 mm is closer to 3.54 inches. Formatted capacities are generally set in terms of kilobytes and megabytes.

Data is generally written to floppy disks in sectors (angular blocks) and tracks (concentric rings at a constant radius). For example, the HD format of 3½-inch floppy disks uses 512 bytes per sector, 18 sectors per track, 80 tracks per side and two sides, for a total of 1,474,560 bytes per disk. Some disk controllers can vary these parameters at the user's request, increasing storage on the disk, although they may not be able to be read on machines with other controllers. For example, Microsoft applications were often distributed on 3½-inch 1.68 MB DMF disks formatted with 21 sectors instead of 18; they could still be recognized by a standard controller. On the IBM PC, MSX and most other microcomputer platforms, disks were written using a constant angular velocity (CAV) format, with the disk spinning at a constant speed and the sectors holding the same amount of information on each track regardless of radial location.

Because the sectors have constant angular size, the 512 bytes in each sector are compressed more near the disk's center. A more space-efficient technique would be to increase the number of sectors per track toward the outer edge of the disk, from 18 to 30 for instance, thereby keeping nearly constant the amount of physical disk space used for storing each sector; an example is zone bit recording. Apple implemented this in early Macintosh computers by spinning the disk more slowly when the head was at the edge, while maintaining the data rate, allowing 400 KB of storage per side and an extra 80 KB on a double-sided disk. This higher capacity came with a disadvantage: the format used a unique drive mechanism and control circuitry, meaning that Mac disks could not be read on other computers. Apple eventually reverted to constant angular velocity on HD floppy disks with their later machines, still unique to Apple as they supported the older variable-speed formats.

Disk formatting is usually done by a utility program supplied by the computer OS manufacturer; generally, it sets up a file storage directory system on the disk, and initializes its sectors and tracks. Areas of the disk unusable for storage due to flaws can be locked (marked as "bad sectors") so that the operating system does not attempt to use them. This was time-consuming so many environments had quick formatting which skipped the error checking process. When floppy disks were often used, disks pre-formatted for popular computers were sold. The unformatted capacity of a floppy disk does not include the sector and track headings of a formatted disk; the difference in storage between them depends on the drive's application. Floppy disk drive and media manufacturers specify the unformatted capacity (for example, 2 MB for a standard 3½-inch HD floppy). It is implied that this should not be exceeded, since doing so will most likely result in performance problems. DMF was introduced permitting 1.68 MB to fit onto an otherwise standard 3½-inch disk; utilities then appeared allowing disks to be formatted as such.

Mixtures of decimal prefixes and binary sector sizes require care to properly calculate total capacity. Whereas semiconductor memory naturally favors powers of two (size doubles each time an address pin is added to the integrated circuit), the capacity of a disk drive is the product of sector size, sectors per track, tracks per side and sides (which in hard disk drives with multiple platters can be greater than 2). Although other sector sizes have been known in the past, formatted sector sizes are now almost always set to powers of two (256 bytes, 512 bytes, etc.), and, in some cases, disk capacity is calculated as multiples of the sector size rather than only in bytes, leading to a combination of decimal multiples of sectors and binary sector sizes. For example, 1.44 MB 3½-inch HD disks have the "M" prefix peculiar to their context, coming from their capacity of 2,880 512-byte sectors (1,440 KiB), consistent with neither a decimal megabyte nor a binary mebibyte (MiB). Hence, these disks hold 1.47 MB or 1.41 MiB. Usable data capacity is a function of the disk format used, which in turn is determined by the FDD controller and its settings. Differences between such formats can result in capacities ranging from approximately 1,300 to 1,760 KiB (1.80 MB) on a standard 3½-inch high-density floppy (and up to nearly 2 MB with utilities such as 2M/2MGUI). The highest capacity techniques require much tighter matching of drive head geometry between drives, something not always possible and unreliable. For example, the LS-240 drive supports a 32 MB capacity on standard 3½-inch HD disks, but this is a write-once technique, and requires its own drive.

The raw maximum transfer rate of 3½-inch ED floppy drives (2.88 MB) is nominally 1,000 kilobits/s, or approximately 83% that of single-speed CD‑ROM (71% of audio CD). This represents the speed of raw data bits moving under the read head; however, the effective speed is somewhat less due to space used for headers, gaps and other format fields and can be even further reduced by delays to seek between tracks.

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

title: Floppy disk


text:
Fencing

Fencing is a combat sport that features sword fighting. The three disciplines of modern fencing are the foil, the épée, and the sabre (also saber); each discipline uses a different kind of blade, which shares the same name, and employs its own rules. Most competitive fencers specialise in one discipline. The modern sport gained prominence near the end of the 19th century and is based on the traditional skill set of swordsmanship. The Italian school altered the historical European martial art of classical fencing, and the French school later refined that system. Scoring points in a fencing competition is done by making contact with an opponent.

The 1904 Olympics Games featured a fourth discipline of fencing known as singlestick, but it was dropped after that year and is not a part of modern fencing. Competitive fencing was one of the first sports to be featured in the Olympics and, along with athletics, cycling, swimming, and gymnastics, has been featured in every modern Olympics.

Governing body
Fencing is governed by the Fédération Internationale d'Escrime (FIE), headquartered in Lausanne, Switzerland. The FIE is composed of 155 national federations, each of which is recognised by its state Olympic Committee as the sole representative of Olympic-style fencing in that country.

Rules
The FIE maintains the current rules used by major international events, including world cups, world championships and the Olympic Games. The FIE handles proposals to change the rules at an annual congress.

Universities and schools
University students compete internationally at the World University Games. The United States holds two national-level university tournaments (the NCAA championship and the USACFC National Championships). The BUCS holds fencing tournaments in the United Kingdom. Many universities in Ontario, Canada have fencing teams that participate in an annual inter-university competition called the OUA Finals.

National fencing organisations have set up programmes to encourage more students to fence. Examples include the Regional Youth Circuit program in the US and the Leon Paul Youth Development series in the UK.

The UK hosts two national competitions in which schools compete against each other directly: the Public Schools Fencing Championship, a competition only open to Independent Schools, and the Scottish Secondary Schools Championships, open to all secondary schools in Scotland. It contains both teams and individual events and is highly anticipated. Schools organise matches directly against one another and school age pupils can compete individually in the British Youth Championships.

In recent years, attempts have been made to introduce fencing to a wider and younger audience, by using foam and plastic swords, which require much less protective equipment. This makes it much less expensive to provide classes, and thus easier to take fencing to a wider range of schools than traditionally has been the case. There is even a competition series in Scotland – the Plastic-and-Foam Fencing FunLeague – specifically for Primary and early Secondary school-age children using this equipment.

History

Fencing traces its roots to the development of swordsmanship for duels and self-defence. The oldest surviving treatise on western fencing is the Royal Armouries Ms. I.33, also known as the Tower manuscript, written c. 1300 in present-day Germany, which discusses the usage of the arming sword together with the buckler. It was followed by a number of treatises, primarily from Germany and Italy, with the oldest surviving Italian treatise being Fior di Battaglia by Fiore dei Liberi, written c. 1400. However, because they were written for the context of a knightly duel with a primary focus on archaic weapons such as the arming sword, longsword, or poleaxe, these older treatises do not really stand in continuity with modern fencing.

From the 16th century onward, the Italian school of fencing would be dominated by the Bolognese or Dardi-School of fencing, named after its founder, Filippo Dardi, a Bolognese fencing master and Professor of Geometry at the University of Bologna. Unlike the previous traditions, the Bolognese school would primarily focus on the sidesword being either used alone or in combination with a buckler, a cape, a parrying dagger, or dual-wielded with another sidesword, though some Bolognese masters, such as Achille Marozo, would still cover the usage of the two-handed greatsword or spadone. The Bolognese school would eventually spread outside of Italy and lay the foundation for modern fencing, eclipsing both older Italian and German traditions. This was partially due to the German schools' focus on archaic weapons such as the longsword, but also due to a general decline in fencing within Germany.

The mechanics of modern fencing originated in the 18th century in an Italian school of fencing of the Renaissance, and under their influence, were improved by the French school of fencing. The Spanish school of fencing stagnated and was replaced by the Italian and French schools.

Development into a sport
The shift towards fencing as a sport rather than as military training happened from the mid-18th century, and was led by Domenico Angelo, who established a fencing academy, Angelo's School of Arms, in Carlisle House, Soho, London in 1763. There, he taught the aristocracy the fashionable art of swordsmanship. His school was run by three generations of his family and dominated the art of European fencing for almost a century.

He established the essential rules of posture and footwork that still govern modern sport fencing, although his attacking and parrying methods were still much different from current practice. Although he intended to prepare his students for real combat, he was the first fencing master to emphasise the health and sporting benefits of fencing more than its use as a killing art, particularly in his influential book L'École des armes (The School of Fencing), published in 1763.

Basic conventions were collated and set down during the 1880s by the French fencing master Camille Prévost. It was during this time that many officially recognised fencing associations began to appear in different parts of the world, such as the Amateur Fencers League of America was founded in 1891, the Amateur Fencing Association of Great Britain in 1902, and the Fédération Nationale des Sociétés d’Escrime et Salles d’Armes de France in 1906.

The first regularised fencing competition was held at the inaugural Grand Military Tournament and Assault at Arms in 1880, held at the Royal Agricultural Hall, in Islington in June. The Tournament featured a series of competitions between army officers and soldiers. Each bout was fought for five hits and the foils were pointed with black to aid the judges. The Amateur Gymnastic & Fencing Association drew up an official set of fencing regulations in 1896.

Fencing was part of the Olympic Games in the summer of 1896. Sabre events have been held at every Summer Olympics; foil events have been held at every Summer Olympics except 1908; épée events have been held at every Summer Olympics except in the summer of 1896 because of unknown reasons.

Starting with épée in 1933, side judges were replaced by the Laurent-Pagan electrical scoring apparatus, with an audible tone and a red or green light indicating when a touch landed. Foil was automated in 1956, sabre in 1988. The scoring box reduced the bias in judging, and permitted more accurate scoring of faster actions, lighter touches, and more touches to the back and flank than before.

Weapons
Each of the three weapons in fencing has its own rules and strategies.

Foil

The foil is a light thrusting weapon with a maximum weight of 500 grams. The foil targets the torso, but not the arms or legs. The foil has a small circular hand guard that serves to protect the hand from direct stabs. As the hand is not a valid target in foil, this is primarily for safety. Touches are scored only with the tip; hits with the side of the blade do not register on the electronic scoring apparatus (and do not halt the action). Touches that land outside the target area (called an off-target touch and signalled by a distinct color on the scoring apparatus) stop the action, but are not scored. Only a single touch can be awarded to either fencer at the end of a phrase. If both fencers land touches within 300 ms (± 25 ms tolerance)Material rules to register two lights on the machine, the referee uses the rules of "right of way" to determine which fencer is awarded the touch, or if an off-target hit has priority over a valid hit, in which case no touch is awarded. If the referee is unable to determine which fencer has right of way, no touch is awarded.

Épée

The épée is a thrusting weapon like the foil, but heavier, with a maximum total weight of 775 grams. In épée, the entire body is a valid target. The hand guard on the épée is a large circle that extends towards the pommel, effectively covering the hand, which is a valid target in épée. Like foil, all hits must be with the tip and not the sides of the blade. Hits with the side of the blade do not register on the electronic scoring apparatus (and do not halt the action). As the entire body is a legal target, there is no concept of an off-target touch, except if the fencer accidentally strikes the floor, setting off the light and tone on the scoring apparatus. Unlike foil and sabre, épée does not use "right of way", simultaneous touches to both fencers, known as "double touches." However, if the score is tied in a match at the last point and a double touch is scored, the point is null and void.

Sabre

The sabre is a light cutting and thrusting weapon that targets the entire body above the waist, including the head and both the hands. Sabre is the newest weapon to be used. Like the foil, the maximum legal weight of a sabre is 500 grams. The hand guard on the sabre extends from hilt to the point at which the blade connects to the pommel. This guard is generally turned outwards during sport to protect the sword arm from touches. Hits with the entire blade or point are valid. As in foil, touches that land outside the target area are not scored. However, unlike foil, these off-target touches do not stop the action, and the fencing continues. In the case of both fencers landing a scoring touch, the referee determines which fencer receives the point for the action, again through the use of "right of way".

Protective clothing
Most personal protective equipment for fencing is made of tough cotton or nylon. Kevlar was added to top level uniform pieces (jacket, breeches, underarm protector, lamé, and the bib of the mask) following the death of Vladimir Smirnov at the 1982 World Championships in Rome. However, Kevlar is degraded by both ultraviolet light and chlorine, which can complicate cleaning.

Other ballistic fabrics, such as Dyneema, have been developed that resist puncture, and which do not degrade the way that Kevlar does. FIE rules state that tournament wear must be made of fabric that resists a force of 800 N, and that the mask bib must resist twice that amount.

The complete fencing kit includes:

 * Jacket
 * The jacket is form-fitting, and has a strap (croissard) that passes between the legs. In sabre fencing, jackets are cut along the waist. A small gorget of folded fabric is sewn in around the collar to prevent an opponent's blade from slipping under the mask and along the jacket upwards towards the neck. Fencing instructors may wear a heavier jacket, such as one reinforced by plastic foam, to deflect the frequent hits an instructor endures.

 * Plastron
 * A plastron is an underarm protector worn underneath the jacket. It provides double protection on the side of the sword arm and upper arm. There is no seam under the arm, which would line up with the jacket seam and provide a weak spot.

 * Glove
 * The sword hand is protected by a glove with a gauntlet that prevents blades from going up the sleeve and causing injury. The glove also improves grip.

 * Breeches
 * Breeches or knickers are short trousers that end just below the knee. The breeches are required to have 10 cm of overlap with the jacket.  Most are equipped with suspenders (braces).

 * Socks
 * Fencing socks are long enough to cover the knee; some cover most of the thigh.

 * Shoes
 * Fencing shoes have flat soles, and are reinforced on the inside for the back foot, and in the heel for the front foot. The reinforcement prevents wear from lunging.

 * Mask
 * The fencing mask has a bib that protects the neck. The mask should support 12 kg on the metal mesh and 350 N of penetration resistance on the bib. FIE regulations dictate that masks must withstand 25 kg on the mesh and 1600 N on the bib. Some modern masks have a see-through visor in the front of the mask. These have been used at high level competitions (World Championships etc.), however, they are currently banned in foil and épée by the FIE, following a 2009 incident in which a visor was pierced during the European Junior Championship competition. There are foil, sabre, and three-weapon masks.

 * Chest protector
 * A chest protector, made of plastic, is worn by female fencers and, sometimes, by males. Fencing instructors also wear them, as they are hit far more often during training than their students. In foil fencing, the hard surface of a chest protector decreases the likelihood that a hit registers.

 * Lamé
 * A lamé is a layer of electrically conductive material worn over the fencing jacket in foil and sabre fencing. The lamé covers the entire target area, and makes it easier to determine whether a hit fell within the target area. (In épée fencing the lamé is unnecessary, since the target area spans the competitor's entire body.) In sabre fencing, the lamé's sleeves end in a straight line across the wrist; in foil fencing, the lamé is sleeveless. A body cord is necessary to register scoring. It attaches to the weapon and runs inside the jacket sleeve, then down the back and out to the scoring box. In sabre and foil fencing, the body cord connects to the lamé in order to create a circuit to the scoring box.

 * Sleeve
 * An instructor or master may wear a protective sleeve or a leg leather to protect their fencing arm or leg, respectively.

Traditionally, the fencer's uniform is white, and an instructor's uniform is black. This may be due to the occasional pre-electric practice of covering the point of the weapon in dye, soot, or coloured chalk in order to make it easier for the referee to determine the placing of the touches. As this is no longer a factor in the electric era, the FIE rules have been relaxed to allow coloured uniforms (save black). The guidelines also limit the permitted size and positioning of sponsorship logos.

Grips
Some pistol grips used by foil and épée fencers

Electric equipment
A set of electric fencing equipment is required to participate in electric fencing. Electric equipment in fencing varies depending on the weapon with which it is used in accordance. The main component of a set of electric equipment is the body cord. The body cord serves as the connection between a fencer and a reel of wire that is part of a system for electrically detecting that the weapon has touched the opponent. There are two types: one for épée, and one for foil and sabre.

Épée body cords consist of two sets of three prongs each connected by a wire. One set plugs into the fencer's weapon, with the other connecting to the reel. Foil and sabre body cords have only two prongs (or a twist-lock bayonet connector) on the weapon side, with the third wire connecting instead to the fencer's lamé. The need in foil and sabre to distinguish between on and off-target touches requires a wired connection to the valid target area.

A body cord consists of three wires known as the A, B, and C lines. At the reel connector (and both connectors for Épée cords) The B pin is in the middle, the A pin is 1.5 cm to one side of B, and the C pin is 2 cm to the other side of B. This asymmetrical arrangement ensures that the cord cannot be plugged in the wrong way around.

In foil, the A line is connected to the lamé and the B line runs up a wire to the tip of the weapon. The B line is normally connected to the C line through the tip. When the tip is depressed, the circuit is broken and one of three things can happen: In Épée, the A and B lines run up separate wires to the tip (there is no lamé). When the tip is depressed, it connects the A and B lines, resulting in a valid touch. However, if the tip is touching the opponents weapon (their C line) or the grounded strip, nothing happens when it is depressed, as the current is redirected to the C line. Grounded strips are particularly important in Épée, as without one, a touch to the floor registers as a valid touch (rather than off-target as in Foil).
 * The tip is touching the opponent's lamé (their A line): Valid touch
 * The tip is touching the opponent's weapon or the grounded strip: nothing, as the current is still flowing to the C line.
 * The tip is not touching either of the above: Off-target hit (white light).

In Sabre, similarly to Foil, the A line is connected to the lamé, but both the B and C lines are connected to the body of the weapon. Any contact between one's B/C line (either one, as they are always connected) and the opponent's A line (their lamé) results in a valid touch. There is no need for grounded strips in Sabre, as hitting something other than the opponent's lame does nothing. In a professional fencing competition, a complete set of electric equipment is needed.

A complete set of foil electric equipment includes: The electric equipment of sabre is very similar to that of foil. In addition, equipment used in sabre includes: Épée fencers lack a lamé, conductive bib, and head cord due to their target area. Also, their body cords are constructed differently as described above. However, they possess all of the other components of a foil fencer's equipment.
 * An electric body cord, which runs under the fencer's jacket on his/her dominant side.
 * An electric blade.
 * A conductive lamé or electric vest.
 * A conductive bib (often attached to the mask).
 * An electric mask cord, connecting the conductive bib and the lamé.
 * A larger conductive lame.
 * An electric sabre.
 * A completely conductive mask.
 * A conductive glove or overlay.

Techniques
Techniques or movements in fencing can be divided into two categories: offensive and defensive. Some techniques can fall into both categories (e.g. the beat). Certain techniques are used offensively, with the purpose of landing a hit on one's opponent while holding the right of way (foil and sabre). Others are used defensively, to protect against a hit or obtain the right of way.

The attacks and defences may be performed in countless combinations of feet and hand actions. For example, fencer A attacks the arm of fencer B, drawing a high outside parry; fencer B then follows the parry with a high line riposte. Fencer A, expecting that, then makes his own parry by pivoting his blade under fencer B's weapon (from straight out to more or less straight down), putting fencer B's tip off target and fencer A now scoring against the low line by angulating the hand upwards.

Offensive

 * Attack: A basic fencing technique, also called a thrust, consisting of the initial offensive action made by extending the arm and continuously threatening the opponent's target. There are four different attacks (straight thrust, disengage attack, counter-disengage attack and cutover). In sabre, attacks are also made with a cutting action.
 * Riposte: An attack by the defender after a successful parry.  After the attacker has completed their attack, and it has been parried, the defender then has the opportunity to make an attack, and (at foil and sabre) take right of way.
 * Feint: A false attack with the purpose of provoking a reaction from the opposing fencer.
 * Lunge: A thrust while extending the front leg by using a slight kicking motion and propelling the body forward with the back leg.
 * Beat attack: In foil and sabre, the attacker beats the opponent's blade to gain priority (right of way) and continues the attack against the target area. In épée, a similar beat is made but with the intention to disturb the opponent's aim and thus score with a single light.
 * Disengage: A blade action whereby the blade is moved around the opponent's blade to threaten a different part of the target or deceive a parry.
 * Compound attack: An attack preceded by one or more feints which oblige the opponent to parry, allowing the attacker to deceive the parry.
 * Continuation/renewal of Attack: A typical épée action of making a 2nd attack after the first attack is parried. This may be done with a change in line; for example, an attack in the high line (above the opponent's bell guard, such as the shoulder) is then followed with an attack to the low line (below the opponent's bell guard, such as the thigh, or foot); or from the outside line (outside the bell guard, such as outer arm) to the inside line (inside the bell guard, such as the inner arm or the chest). A second continuation is stepping slight past the parry and angulating the blade to bring the tip of the blade back on target. A renewal may also be direct (without a change of line or any further blade action), in which case it is called a remise. In foil or sabre, a renewal is considered to have lost right of way, and the defender's immediate riposte, if it lands, will score instead of the renewal.
 * Flick: a technique used primarily in foil and épée. It takes advantage of the extreme flexibility of the blade to use it like a whip, bending the blade so that it curves over and strikes the opponent with the point; this allows the fencer to hit an obscured part of the target (e.g., the back of the shoulder or, at épée, the wrist even when it is covered by the guard).  This technique has become much more difficult due to timing changes which require the point to stay depressed for longer to set off the light.
 * Flèche: an offensive manoeuvre, in which the fencer leans forward past the point of balance, and then crosses their feet, running past the opponent after the touch is scored.
 * Flunge: a technique used by sabreurs in which the attacker makes a flying lunge, as FIE rules state that crossing one's feet is illegal in Sabre.

Defensive

 * Parry: Basic defence technique, block the opponent's weapon while it is preparing or executing an attack to deflect the blade away from the fencer's valid area and (in foil and sabre) to give fencer the right of way. Usually followed by a riposte, a return attack by the defender.
 * Circle parry: A parry where the weapon is moved in a circle to catch the opponent's tip and deflect it away.
 * Counter attack: A basic fencing technique of attacking one's opponent while generally moving back out of the way of the opponent's attack. Used quite often in épée to score against the attacker's hand/arm. More difficult to accomplish in foil and sabre unless one is quick enough to make the counterattack and retreat ahead of the advancing opponent without being scored upon, or by evading the attacking blade via moves such as the In Quartata (turning to the side) or Passata-sotto (ducking). Counterattacks can also be executed in opposition, grazing along the opponent's blade and deflecting it to cause the attack to miss.
 * Point-in-line: A specific position where the arm is straight and the point is threatening the opponent's target area. In foil and sabre, this gives one priority if the extension is completed before the opponent begins the final action of their attack.  When performed as a defensive action, the attacker must then disturb the extended weapon to re-take priority; otherwise the defender has priority and the point-in-line will win the touch if the attacker does not manage a single light.  In épée, there is no priority; the move may be used as a means by either fencer to achieve a double-touch and advance the score by 1 for each fencer. In all weapons, the point-in-line position is commonly used to slow the opponent's advance and cause them to delay the execution of their attack.

Other variants
Other variants include wheelchair fencing for those with disabilities, chair fencing, one-hit épée (one of the five events which constitute modern pentathlon) and the various types of non-Olympic competitive fencing. Chair fencing is similar to wheelchair fencing, but for the able bodied. The opponents set up opposing chairs and fence while seated; all the usual rules of fencing are applied. An example of the latter is the American Fencing League (distinct from the United States Fencing Association): the format of competitions is different and the right of way rules are interpreted in a different way. In a number of countries, school and university matches deviate slightly from the FIE format. A variant of the sport using toy lightsabers earned national attention when ESPN2 acquired the rights to a selection of matches and included it as part of its "ESPN8: The Ocho" programming block in August 2018.

Two handed fencing refers to a type of fencing where a buckler or dagger is used to parry attacks.

In popular culture
One of the most notable films related to fencing is the 2015 Finnish-Estonian-German film The Fencer, directed by Klaus Härö, which is loosely based on the life of Endel Nelis, an accomplished Estonian fencer and coach. The film was nominated for the 73rd Golden Globe Awards in the Foreign Language Film Category.

In 2017, the first issue of the Fence comic book series, which follows a fictional team of young fencers, was published by the US-based Boom! Studios.

title: Fencing


text:
The Free Software Definition

The Free Software Definition written by Richard Stallman and published by the Free Software Foundation (FSF), defines free software as being software that ensures that the users have freedom in using, studying, sharing and modifying that software. The term "free" is used in the sense of "free speech," not of "free of charge." The earliest-known publication of the definition was in the February 1986 edition of the now-discontinued GNU's Bulletin publication by the FSF. The canonical source for the document is in the philosophy section of the GNU Project website. , it is published in 39 languages. The FSF publishes a list of licences that meet this definition.

The Four Essential Freedoms of Free Software
The definition published by the FSF in February 1986 had two points:

"The word 'free' in our name does not refer to price; it refers to freedom. First, the freedom to copy a program and redistribute it to your neighbors, so that they can use it as well as you. Second, the freedom to change a program, so that you can control it instead of it controlling you; for this, the source code must be made available to you."

In 1996, when the gnu.org website was launched, "free software" was defined referring to "three levels of freedom" by adding an explicit mention of the freedom to study the software (which could be read in the two-point definition as being part of the freedom to change the program). Stallman later avoided the word "levels", saying that all of the freedoms are needed, so it is misleading to think in terms of levels.

Finally, another freedom was added, to explicitly say that users should be able to run the program. The existing freedoms were already numbered one to three, but this freedom should come before the others, so it was added as "freedom zero".

The modern definition defines free software by whether or not the recipient has the following four freedoms:

"*The freedom to run the program as you wish, for any purpose (freedom 0).
 * The freedom to study how the program works, and change it so it does your computing as you wish (freedom 1). Access to the source code is a precondition for this.
 * The freedom to redistribute copies so you can help your neighbor (freedom 2).
 * The freedom to distribute copies of your modified versions to others (freedom 3). By doing this you can give the whole community a chance to benefit from your changes. Access to the source code is a precondition for this."

Freedoms 1 and 3 require source code to be available because studying and modifying software without its source code is highly impractical.

Later definitions
In July 1997, Bruce Perens published the Debian Free Software Guidelines. A definition based on the DFSG was also used by the Open Source Initiative (OSI) under the name "The Open Source Definition".

Comparison with The Open Source Definition
Despite the philosophical differences between the free software movement and the open-source-software movement, the official definitions of free software by the FSF and of open-source software by the OSI basically refer to the same software licences, with a few minor exceptions. While stressing these philosophical differences, the Free Software Foundation comments: "The term "open source" software is used by some people to mean more or less the same category as free software. It is not exactly the same class of software: they accept some licences that we consider too restrictive, and there are free software licences they have not accepted. However, the differences in extension of the category are small: nearly all free software is open source, and nearly all open source software is free."

- Free Software Foundation

title: The Free Software Definition