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Contract bridge

Contract bridge, or simply bridge, is a trick-taking card game using a standard 52-card deck. In its basic format, it is played by four players in two competing partnerships, with partners sitting opposite each other around a table. Millions of people play bridge worldwide in clubs, tournaments, online and with friends at home, making it one of the world's most popular card games, particularly among seniors. The World Bridge Federation (WBF) is the governing body for international competitive bridge, with numerous other bodies governing it at the regional level.

The game consists of a number of, each progressing through four phases. The cards are to the players; then the players call (or bid) in an  seeking to take the, specifying how many tricks the partnership receiving the contract (the declaring side) needs to take to receive points for the deal. During the auction, partners use their bids to exchange information about their hands, including overall strength and distribution of the suits; no other means of conveying or implying any information is permitted. The cards are then played, the trying to fulfill the contract, and the  trying to stop the declaring side from achieving its goal. The deal is scored based on the number of tricks taken, the contract, and various other factors which depend to some extent on the variation of the game being played.

Rubber bridge is the most popular variation for casual play, but most club and tournament play involves some variant of duplicate bridge, where the cards are not re-dealt on each occasion, but the same deal is played by two or more sets of players (or "tables") to enable comparative scoring.

History and etymology
Bridge is a member of the family of trick-taking games and is a derivative of whist, which had become the dominant such game and enjoyed a loyal following for centuries. The idea of a trick-taking 52-card game has its first documented origins in Italy and France. The French physician and author Rabelais (1493–1553) mentions a game called "La Triomphe" in one of his works. Also Juan Luis Vives, in his Linguae latinae exercitio (Exercise in the Latin language) of 1539 has a dialogue on card games, where the characters play 'Triumphus hispanicus' (Spanish Triumph).

Bridge departed from whist with the creation of "Biritch" in the 19th century and evolved through the late 19th and early 20th centuries to form the present game. The first rule book for bridge, dated 1886, is Biritch, or Russian Whist written by John Collinson, an English financier working in Ottoman Constantinople. It and his subsequent letter to The Saturday Review dated 28 May 1906, document the origin of Biritch as being the Russian community in Constantinople. The word biritch is thought to be a transliteration of the Russian word Бирюч (бирчий, бирич), an occupation of a diplomatic clerk or an announcer. Another theory is that British soldiers invented the game bridge while serving in the Crimean War, and named it after the Galata Bridge, which they crossed on their way to a coffeehouse to play cards.

Biritch had many significant bridge-like developments: dealer chose the trump suit, or nominated his partner to do so; there was a call of "no trumps" (biritch); dealer's partner's hand became dummy; points were scored above and below the line; game was 3NT, 4 and 5 (although 8 club odd tricks and 15 spade odd tricks were needed); the score could be doubled and redoubled; and there were slam bonuses. It has some features in common with solo whist. This game, and variants of it known as "bridge" and "bridge whist", became popular in the United States and the United Kingdom in the 1890s despite the long-established dominance of whist. Its breakthrough was its acceptance in 1894 by Lord Brougham at London's Portland Club.

In 1904 auction bridge was developed, in which the players bid in a competitive auction to decide the contract and declarer. The object became to make at least as many tricks as were contracted for, and penalties were introduced for failing to do so. Auction bridge bidding beyond winning the auction is pointless. If taking all 13 tricks, there is no difference in score between a 1 and a 7 final bid, as the bonus for rubber, small slam or grand slam depends on the number of tricks taken rather than the number of tricks bid.

The modern game of contract bridge was the result of innovations to the scoring of auction bridge by Harold Stirling Vanderbilt and others. The most significant change was that only the tricks contracted for were scored below the line toward game or a slam bonus, a change that resulted in bidding becoming much more challenging and interesting. Also new was the concept of "vulnerability", making sacrifices to protect the lead in a rubber more expensive. The various scores were adjusted to produce a more balanced and interesting game. Vanderbilt set out his rules in 1925, and within a few years contract bridge had so supplanted other forms of the game that "bridge" became synonymous with "contract bridge".

The form of bridge mostly played in clubs, tournaments and online is duplicate bridge. The number of people playing contract bridge has declined since its peak in the 1940s, when a survey found it was played in 44% of US households. The game is still widely played, especially amongst retirees, and in 2005 the ACBL estimated there were 25 million players in the US.

Overview
Bridge is a four-player partnership trick-taking game with thirteen tricks per deal. The dominant variations of the game are rubber bridge, more common in social play; and duplicate bridge, which enables comparative scoring in tournament play. Each player is dealt thirteen cards from a standard 52-card deck. A starts when a player leads, i.e. plays the first card. The leader to the first trick is determined by the auction; the leader to each subsequent trick is the player who won the preceding trick. Each player, in clockwise order, plays one card on the trick. Players must play a card of the same suit as the original card led, unless they have none (said to be "void"), in which case they may play any card.

The player who played the highest-ranked card wins the trick. Within a suit, the ace is ranked highest followed by the king, queen and jack and then the ten through to the two. In a deal where the auction has determined that there is no trump suit, the trick must be won by a card of the suit led. In a deal with a trump suit, cards of that suit are superior in rank to any of the cards of any other suit. If one or more players plays a trump to a trick when void in the suit led, the highest trump wins. For example, if the trump suit is spades and a player is void in the suit led and plays a spade card, they win the trick if no other player plays a higher spade. If a trump suit is led, the usual rule for trick-taking applies.

Unlike its predecessor, whist, the goal of bridge is not simply to take the most tricks in a deal. Instead, the goal is to successfully estimate how many tricks one's partnership can take. To illustrate this, the simpler partnership trick-taking game of spades has a similar mechanism: the usual trick-taking rules apply with the trump suit being spades, but in the beginning of the game, players bid or estimate how many tricks they can win, and the number of tricks bid by both players in a partnership are added. If a partnership takes at least that many tricks, they receive points for the round; otherwise, they lose penalty points.

Bridge extends the concept of bidding into an, where partnerships compete to take a , specifying how many tricks they will need to take in order to receive points, and also specifying the trump suit (or no trump, meaning that there will be no trump suit). Players take turns to call in a clockwise order: each player in turn either passes, doubles – which increases the penalties for not making the contract specified by the opposing partnership's last bid, but also increases the reward for making it – or redoubles, or states a contract that their partnership will adopt, which must be higher than the previous highest bid (if any). Eventually, the player who bid the highest contract – which is determined by the contract's level as well as the trump suit or no trump – wins the contract for their partnership.

In the example auction below, the east–west pair secures the contract of 6; the auction concludes when there have been three successive passes. Note that six tricks are added to contract values, so the six-level contract is a contract of twelve tricks. In practice, establishing a contract without enough information on the other partner's hand is difficult, so there exist many bidding systems assigning meanings to bids, with common ones including Standard American, Acol, and 2/1 game forcing. Contrast with Spades, where players only have to bid their own hand.

After the contract is decided, and the first lead is made, the declarer's partner (dummy) lays their cards face up on the table, and the declarer plays the dummy's cards as well as their own. The opposing partnership is called the, and their goal is to stop the declarer from fulfilling his contract. Once all the cards have been played, the hand is scored: if the declaring side makes their contract, they receive points based on the level of the contract, with some trump suits being worth more points than others and no trump being the highest, as well as bonus points for. If the declarer fails to fulfill the contract, the defenders receive points depending on the declaring side's undertricks (the number of tricks short of the contract) and whether the contract was doubled by the defenders.

Setup and dealing
The four players sit in two partnerships with players sitting opposite their partners. A cardinal direction is assigned to each seat, so that one partnership sits in North and South, while the other sits in West and East. The cards may be freshly dealt or, in duplicate bridge games, pre-dealt. All that is needed in basic games are the cards and a method of keeping score, but there is often other equipment on the table, such as a board containing the cards to be played (in duplicate bridge), bidding boxes, or screens.

In rubber bridge each player draws a card at the start of the game; the player who draws the highest card deals first. The second highest card becomes the dealer's partner and takes the chair on the opposite side of the table. They play against the other two. The deck is shuffled and cut, usually by the player to the left of the dealer, before dealing. Players take turns to deal, in clockwise order. The dealer deals the cards clockwise, one card at a time. Normally, rubber bridge is played with two packs of cards and whilst one pack is being dealt, the dealer's partner shuffles the other pack. After shuffling the pack is placed on the right ready for the next dealer. Before dealing, the next dealer passes the cards to the previous dealer who cuts them.

In duplicate bridge the cards are pre-dealt, either by hand or by a computerized dealing machine, in order to allow for competitive scoring. Once dealt, the cards are placed in a device called a "board", having slots designated for each player's cardinal direction seating position. After a deal has been played, players return their cards to the appropriate slot in the board, ready to be played by the next table.

Auction
The dealer opens the auction and can make the first call, and the auction proceeds clockwise. When it is their turn to call, a player may pass – but can enter into the bidding later – or bid a contract, specifying the level of their contract and either the trump suit or no trump (the denomination), provided that it is higher than the last bid by any player, including their partner. All bids promise to take a number of tricks in excess of six, so a bid must be between one (seven tricks) and seven (thirteen tricks). A bid is higher than another bid if either the level is greater (e.g., 2 over 1NT) or the denomination is higher, with the order being in ascending (or alphabetical) order:, , , , and NT (no trump). Calls may be made orally or with a bidding box.

If the last bid was by the opposing partnership, one may also the opponents' bid, increasing the penalties for undertricks, but also increasing the reward for making the contract. Doubling does not carry to future bids by the opponents unless future bids are doubled again. A player on the opposing partnership being doubled may also, which increases the penalties and rewards further. Players may not see their partner's hand during the auction, only their own. There exist many bidding conventions that assign agreed meanings to various calls to assist players in reaching an optimal contract (or obstruct the opponents).

The auction ends when, after a player bids, doubles, or redoubles, every other player has passed, in which case the action proceeds to the play; or every player has passed and no bid has been made, in which case the round is considered to be "passed out" and not played.

Play
The player from the declaring side who first bid the denomination named in the final contract becomes declarer. The player left to the declarer leads to the first trick. Dummy then lays his or her cards face-up on the table, organized in columns by suit. Play proceeds clockwise, with each player required to follow suit if possible. Tricks are won by the highest trump, or if there were none played, the highest card of the led suit. The player who won the previous trick leads to the next trick. The declarer has control of the dummy's cards and tells his partner which card to play at dummy's turn. There also exist conventions that communicate further information between defenders about their hands during the play.

At any time, a player may, stating that their side will win a specific number of the remaining tricks. The claiming player lays his cards down on the table and explains the order in which he intends to play the remaining cards. The opponents can either accept the claim and the round is scored accordingly, or dispute the claim. If the claim is disputed, play continues with the claiming player's cards face up in rubber games, or in duplicate games, play ceases and the tournament director is called to adjudicate the hand.

Scoring
At the end of the hand, points are awarded to the declaring side if they make the contract, or else to the defenders. Partnerships can be, increasing the rewards for making the contract, but also increasing the penalties for undertricks. In rubber bridge, if a side has won 100 contract points, they have won a and are vulnerable for the remaining rounds, but in duplicate bridge, vulnerability is predetermined based on the number of each board.

If the declaring side makes their contract, they receive points for, or tricks bid and made in excess of six. In both rubber and duplicate bridge, the declaring side is awarded 20 points per odd trick for a contract in clubs or diamonds, and 30 points per odd trick for a contract in hearts or spades. For a contract in notrump, the declaring side is awarded 40 points for the first odd trick and 30 points for the remaining odd tricks. Contract points are doubled or quadrupled if the contract is respectively doubled or redoubled.

In rubber bridge, a partnership wins one game once it has accumulated 100 contract points; excess contract points do not carry over to the next game. A partnership that wins two games wins the rubber, receiving a bonus of 500 points if the opponents have won a game, and 700 points if they have not.

Overtricks score the same number of points per odd trick, although their doubled and redoubled values differ. Bonuses vary between the two bridge variations both in score and in type (for example, rubber bridge awards a bonus for holding a certain combination of high cards), although some are common between the two.

A larger bonus is awarded if the declaring side makes a small slam or grand slam, a contract of 12 or 13 tricks respectively. If the declaring side is not vulnerable, a small slam gets 500 points, and a grand slam 1000 points. If the declaring side is vulnerable, a small slam is 750 points and a grand slam is 1,500.

In rubber bridge, the rubber finishes when a partnership has won two games, but the partnership receiving the most overall points wins the rubber. Duplicate bridge is scored comparatively, meaning that the score for the hand is compared to other tables playing the same cards and match points are scored according to the comparative results: usually either "matchpoint scoring", where each partnership receives 2 points (or 1 point) for each pair that they beat, and 1 point (or 1⁄2 point) for each tie; or IMPs (international matchpoint) scoring, where the number of IMPs varies (but less than proportionately) with the points difference between the teams.

Undertricks are scored in both variations as follows:

Rules
The rules of the game are referred to as the laws as promulgated by various bridge organizations.

Duplicate
The official rules of duplicate bridge are promulgated by the WBF as "The Laws of Duplicate Bridge 2017". The Laws Committee of the WBF, composed of world experts, updates the Laws every 10 years; it also issues a Laws Commentary advising on interpretations it has rendered.

In addition to the basic rules of play, there are many additional rules covering playing conditions and the rectification of irregularities, which are primarily for use by tournament directors who act as referees and have overall control of procedures during competitions. But various details of procedure are left to the discretion of the zonal bridge organisation for tournaments under their aegis and some (for example, the choice of movement) to the sponsoring organisation (for example, the club).

Some zonal organisations of the WBF also publish editions of the Laws. For example, the American Contract Bridge League (ACBL) publishes the Laws of Duplicate Bridge and additional documentation for club and tournament directors.

Rubber
There are no universally accepted rules for rubber bridge, but some zonal organisations have published their own. An example for those wishing to abide by a published standard is The Laws of Rubber Bridge as published by the American Contract Bridge League.

The majority of rules mirror those of duplicate bridge in the bidding and play and differ primarily in procedures for dealing and scoring.

Online
In 2001, the WBF promulgated a set of laws for online play.

Tournaments
Bridge is a game of skill played with randomly dealt cards, which makes it also a game of chance, or more exactly, a tactical game with inbuilt randomness, imperfect knowledge and restricted communication. The chance element is in the deal of the cards; in duplicate bridge some of the chance element is eliminated by comparing results of multiple pairs in identical situations. This is achievable when there are eight or more players, sitting at two or more tables, and the deals from each table are preserved and passed to the next table, thereby duplicating them for the other table(s) of players. At the end of a session, the scores for each deal are compared, and the most points are awarded to the players doing the best with each particular deal. This measures relative skill (but still with an element of luck) because each pair or team is being judged only on the ability to bid with, and play, the same cards as other players.

Duplicate bridge is played in clubs and tournaments, which can gather as many as several hundred players. Duplicate bridge is a mind sport, and its popularity gradually became comparable to that of chess, with which it is often compared for its complexity and the mental skills required for high-level competition. Bridge and chess are the only "mind sports" recognized by the International Olympic Committee, although they were not found eligible for the main Olympic program. In October 2017 the British High Court ruled against the English Bridge Union, finding that Bridge is not a sport under a definition of sport as involving physical activity, but did not rule on the "broad, somewhat philosophical question" as to whether or not bridge is a sport.

The basic premise of duplicate bridge had previously been used for whist matches as early as 1857. Initially, bridge was not thought to be suitable for duplicate competition; it was not until the 1920s that (auction) bridge tournaments became popular.

In 1925 when contract bridge first evolved, bridge tournaments were becoming popular, but the rules were somewhat in flux, and several different organizing bodies were involved in tournament sponsorship: the American Bridge League (formerly the American Auction Bridge League, which changed its name in 1929), the American Whist League, and the United States Bridge Association. In 1935, the first officially recognized world championship was held. In 1958, the World Bridge Federation (WBF) was founded to promote bridge worldwide, coordinate periodic revision to the Laws (each ten years, next in 2027) and conduct world championships.

Bidding boxes and screens
In tournaments, "bidding boxes" are frequently used, as noted above. These avoid the possibility of players at other tables hearing any spoken bids. The bidding cards are laid out in sequence as the auction progresses. Although it is not a formal rule, many clubs adopt a protocol that the bidding cards stay revealed until the first playing card is tabled, after which point the bidding cards are put away. Bidding pads are an alternative to bidding boxes. A bidding pad is a block of 100mm square tear-off sheets. Players write their bids on the top sheet. When the first trick is complete the sheet is torn off and discarded.

In top national and international events, "bidding screens" are used. These are placed diagonally across the table, preventing partners from seeing each other during the game; often the screen is removed after the auction is complete.

Bidding
Much of the complexity in bridge arises from the difficulty of arriving at a good final contract in the auction (or deciding to let the opponents declare the contract). This is a difficult problem: the two players in a partnership must try to communicate enough information about their hands to arrive at a makeable contract, but the information they can exchange is restricted – information may be passed only by the calls made and later by the cards played, not by other means; in addition, the agreed-upon meaning of each call and play must be available to the opponents.

Since a partnership that has freedom to bid gradually at leisure can exchange more information, and since a partnership that can interfere with the opponents' bidding (as by raising the bidding level rapidly) can cause difficulties for their opponents, bidding systems are both informational and strategic. It is this mixture of information exchange and evaluation, deduction, and tactics that is at the heart of bidding in bridge.

A number of basic rules of thumb in bridge bidding and play are summarized as bridge maxims.

Systems and conventions
A bidding system is a set of partnership agreements on the meanings of bids. A partnership's bidding system is usually made up of a core system, modified and complemented by specific conventions (optional customizations incorporated into the main system for handling specific bidding situations) which are pre-chosen between the partners prior to play. The line between a well-known convention and a part of a system is not always clear-cut: some bidding systems include specified conventions by default. Bidding systems can be divided into mainly natural systems such as Acol and Standard American, and mainly artificial systems such as the Precision Club and Polish Club.

Calls are usually considered to be either natural or conventional (artificial). A natural call carries a meaning that reflects the call; a natural bid intuitively showing hand or suit strength based on the level or suit of the bid, and a natural double expressing that the player believes that the opposing partnership will not make their contract. By contrast, a conventional (artificial) call offers and/or asks for information by means of pre-agreed coded interpretations, in which some calls convey very specific information or requests that are not part of the natural meaning of the call. Thus in response to 4NT, a 'natural' bid of 5 would state a preference towards a diamond suit or a desire to play in five diamonds, whereas if the partners have agreed to use the common Blackwood convention, a bid of 5 in the same situation would say nothing about the diamond suit, but would tell the partner that the hand in question contains exactly one ace.

Conventions are valuable in bridge because of the need to pass information beyond a simple like or dislike of a particular suit, and because the limited bidding space can be used more efficiently by adopting a conventional (artificial) meaning for a given call where a natural meaning has less utility, because the information it conveys is not valuable or because the desire to convey that information arises only rarely. The conventional meaning conveys more useful (or more frequently useful) information. There are a very large number of conventions from which players can choose; many books have been written detailing bidding conventions. Well-known conventions include Stayman (to ask the opening 1NT bidder to show any four-card major suit), Jacoby transfers (a request by (usually) the weak hand for the partner to bid a particular suit first, and therefore to become the declarer), and the Blackwood convention (to ask for information on the number of aces and kings held, used in slam bidding situations).

The term preempt refers to a high-level tactical bid by a weak hand, relying upon a very long suit rather than high cards for tricks. Preemptive bids serve a double purpose – they allow players to indicate they are bidding on the basis of a long suit in an otherwise weak hand, which is important information to share, and they also consume substantial bidding space which prevents a possibly strong opposing pair from exchanging information on their cards. Several systems include the use of opening bids or other early bids with weak hands including long (usually six to eight card) suits at the 2, 3 or even 4 or 5 levels as preempts.

Basic natural systems
As a rule, a natural suit bid indicates a holding of at least four (or more, depending on the situation and the system) cards in that suit as an opening bid, or a lesser number when supporting partner; a natural NT bid indicates a balanced hand.

Most systems use a count of high card points as the basic evaluation of the strength of a hand, refining this by reference to shape and distribution if appropriate. In the most commonly used point count system, aces are counted as 4 points, kings as 3, queens as 2, and jacks as 1 point; therefore, the deck contains 40 points. In addition, the distribution of the cards in a hand into suits may also contribute to the strength of a hand and be counted as distribution points. A better than average hand, containing 12 or 13 points, is usually considered sufficient to open the bidding, i.e., to make the first bid in the auction. A combination of two such hands (i.e., 25 or 26 points shared between partners) is often sufficient for a partnership to bid, and generally to make, game in a major suit or notrump (more are usually needed for a minor suit game, as the level is higher).

In natural systems, a 1NT opening bid usually reflects a hand that has a relatively balanced shape (usually between two and four (or less often five) cards in each suit) and a sharply limited number of high card points, usually somewhere between 12 and 18 – the most common ranges use a span of exactly three points (for example, 12–14, 15–17 or 16–18), but some systems use a four-point range, usually 15–18.

Opening bids of three or higher are preemptive bids, i.e., bids made with weak hands that especially favor a particular suit, opened at a high level in order to define the hand's value quickly and to frustrate the opposition. For example, a hand of would be a candidate for an opening bid of 3, designed to make it difficult for the opposing team to bid and find their optimum contract even if they have the bulk of the points. This hand is nearly valueless unless spades are trumps but it contains good enough spades that the penalty for being set should not be higher than the value of an opponent game. The high card weakness makes it likely that the opponents have enough strength to make game themselves.

Openings at the 2 level are either unusually strong (2NT, natural, and 2, artificial) or preemptive, depending on the system. Unusually strong bids communicate an especially high number of points (normally 20 or more) or a high trick-taking potential (normally 8 or more). Also 2 as the strongest (by HCP and by DP+HCP) has become more common, perhaps especially at websites that offer duplicate bridge. Here the 2 opening is used for either hands with a good 6-card suit or longer (max one losing card) and a total of 18 HCP up to 23 total points – or "$7 1/2$NT", like 2NT but with 22–23 HCP. Whilst the 2 opening bid takes care of all hands with 24 points (HCP or with distribution points included) with the only exception of "Gambling 3NT".

Opening bids at the one level are made with hands containing 12–13 points or more and which are not suitable for one of the preceding bids. Using Standard American with 5-card majors, opening hearts or spades usually promises a 5-card suit. Partnerships who agree to play 5-card majors open a minor suit with 4-card majors and then bid their major suit at the next opportunity. This means that an opening bid of 1 or 1 will sometimes be made with only 3 cards in that suit.

Doubles are sometimes given conventional meanings in otherwise mostly natural systems. A natural, or penalty double, is one used to try to gain extra points when the defenders are confident of setting (defeating) the contract. The most common example of a conventional double is the takeout double of a low-level suit bid, implying support for the unbid suits or the unbid major suits and asking partner to choose one of them.

Basic variations
Bidding systems depart from these basic ideas in varying degrees. Standard American, for instance, is a collection of conventions designed to bolster the accuracy and power of these basic ideas, while Precision Club is a system that uses the 1 opening bid for all or almost all strong hands (but sets the threshold for "strong" rather lower than most other systems – usually 16 high card points) and may include other artificial calls to handle other situations (but it may contain natural calls as well). Many experts today use a system called 2/1 game forcing (enunciated as two over one game forcing), which amongst other features adds some complexity to the treatment of the one notrump response as used in Standard American. In the UK, Acol is the most common system; its main features are a weak one notrump opening with 12–14 high card points and several variations for 2-level openings.

There are also a variety of advanced techniques used for hand evaluation. The most basic is the Milton Work point count, (the 4-3-2-1 system detailed above) but this is sometimes modified in various ways, or either augmented or replaced by other approaches such as losing trick count, honor point count, law of total tricks, or Zar Points.

Common conventions and variations within natural systems include:
 * Blackwood (either the original version or Roman Key Card)
 * How the partnership's bidding practices will be varied if their opponents intervene or compete.
 * Point count required for 1 NT opening bid ('mini' 10–12, 'weak' 12–14, 'strong' 15–17 or 16–18)
 * Stayman (together with Blackwood, described as "the two most famous conventions in Bridge". )
 * What types of cue bids (e.g. bidding the opponents' suit) the partnership will play, if any.
 * Whether 1 (and sometimes 1) is 'natural' or 'suspect' (also called 'phoney' or 'short'), signifying an opening hand lacking a notable heart or spade suit
 * Whether an opening bid of 1 and 1 requires a minimum of 4 or 5 cards in the suit (4 or 5 card majors)
 * Whether doubling a contract at the 1, 2 and sometimes higher levels signifies a belief that the opponents' contract will fail and a desire to raise the stakes (a penalty double), or an indication of strength but no biddable suit coupled with a request that partner bid something (a takeout double).
 * Whether doubling or overcalling over opponents' 1NT is natural or conventional. One common artificial agreement is Cappelletti, where 2 is a transfer to be passed or corrected to a major, 2 means both majors and a major shows that suit plus a minor.
 * Whether opening bids at the two level are 'strong' (20+ points) or 'weak' (i.e., pre-emptive with a 6 card suit). (Note: an opening bid of 2 is usually played in otherwise natural systems as conventional, signifying any exceptionally strong hand)
 * Whether the partnership will play Jacoby transfers (bids of 2 and 2 over 1NT or 3 and 3 over 2NT respectively require the 1NT or 2NT bidder to rebid 2 and 2 or 3 and 3), minor suit transfers (bids of 2 and either 2NT or 3 over 1NT respectively require the 1NT bidder to bid 3 and 3) and Texas transfers (bids of 4 and 4 respectively require the 1NT, or 2NT bidder to rebid 4 and 4)
 * Which (if any) bids are forcing and require a response.

Within play, it is also commonly agreed what systems of opening leads, signals and discards will be played:
 * Conventions for the opening lead govern how the first card to be played will be chosen and what it will mean,
 * Count signals cover the situation when a defender is following suit (usually to a suit that the declarer has led). In such circumstances the order in which a defender plays his spot cards will indicate whether an even or odd number of cards was originally held in that suit. This can help the other defender count out the entire original distribution of the cards in that suit. It is sometimes critical to know this when defending.
 * Discards cover the situation when a defender cannot follow suit and therefore has free choice what card to play or throw away. In such circumstances the thrown-away card can be used to indicate some aspect of the hand, or a desire for a specific suit to be played.
 * Signals indicate how cards played within a suit are chosen – for example, playing a noticeably high card when this is unexpected can signal encouragement to continue playing the suit, and a low card can signal discouragement and a desire for partner to choose some other suit. (Some partnerships use "reverse" signals, meaning that a noticeably high card discourages that suit and a noticeably low card encourages that suit, thus not "wasting" a potentially useful intermediate card in the suit of interest.)
 * Suit preference signals cover the situation where a defender is returning a suit which will be ruffed by his partner. If he plays a high card he is showing an entry in the higher side suit and vice versa. There are some other situations where this tool may be used.
 * Surrogate signals cover the situation when it is critical to show length in a side suit and it will be too late if defenders wait until that suit is played. Then, the play in the first declarer played suit is a count signal regarding the critical suit and not the trump suit itself. In fact, any signal made about a suit in another suit might be called as such.

Advanced techniques
Every call (including "pass", also sometimes called "no bid") serves two purposes. It confirms or passes some information to a partner, and, by implication, denies any other kind of hand which would have tended to support an alternative call. For example, a bid of 2NT immediately after partner's 1NT not only shows a balanced hand of a certain point range, but also almost always denies possession of a five-card major suit (otherwise the player would have bid it) or even a four card major suit (in that case, the player should use the Stayman convention).

Likewise, in some partnerships the bid of 2 in the sequence 1NT–2–2–2 between partners (opponents passing throughout) explicitly shows five hearts but also confirms four cards in spades: the bidder must hold at least five hearts to make it worth looking for a heart fit after 2 denied a four card major, and with at least five hearts, a Stayman bid must have been justified by having exactly four spades, the other major (since Stayman (as used by this partnership) is not useful with anything except a four card major suit). Thus an astute partner can read much more than the surface meaning into the bidding. Alternatively, many partnerships play this same bidding sequence as "Crawling Stayman" by which the responder shows a weak hand (less than eight high card points) with shortness in diamonds but at least four hearts and four spades; the opening bidder may correct to spades if that appears to be the better contract.

The situations detailed here are extremely simple examples; many instances of advanced bidding involve specific agreements related to very specific situations and subtle inferences regarding entire sequences of calls.

Play techniques
Terence Reese, a prolific author of bridge books, points out that there are only four ways of taking a trick by force, two of which are very easy:
 * establishing long suits (the last cards in a suit will take tricks if the opponents do not have the suit and are unable to trump)
 * playing a high card that no one else can beat
 * playing for the opponents' high cards to be in a particular position (if their ace is to the right of your king, your king may be able to take a trick, especially if, when that suit is led, the player to your right has to play their card before you do)
 * trumping an opponent's high card

Nearly all trick-taking techniques in bridge can be reduced to one of these four methods. The optimum play of the cards can require much thought and experience and is the subject of whole books on bridge.

Example
The cards are dealt as shown in the bridge hand diagram; North is the dealer and starts the auction which proceeds as shown in the bidding table.

As neither North nor East have sufficient strength to open the bidding, they each pass, denying such strength. South, next in turn, opens with the bid of 1, which denotes a reasonable heart suit (at least 4 or 5 cards long, depending on the bidding system) and at least 12 high card points. On this hand, South has 14 high card points. West overcalls with 1, since he has a long spade suit of reasonable quality and 10 high card points (an overcall can be made on a hand that is not quite strong enough for an opening bid). North supports partner's suit with 2, showing heart support and about 6–8 points. East supports spades with 2. South inserts a game try of 3, inviting the partner to bid the game of 4 with good club support and overall values. North complies, as North is at the higher end of the range for his 2 bid, and has a fourth trump (the 2 bid promised only three), and the doubleton queen of clubs to fit with partner's strength there. (North could instead have bid 3, indicating not enough strength for game, asking South to pass and so play 3.)

In the auction, north–south are trying to investigate whether their cards are sufficient to make a game (nine tricks at notrump, ten tricks in hearts or spades, 11 tricks in clubs or diamonds), which yields bonus points if bid and made. East-West are competing in spades, hoping to play a contract in spades at a low level. 4 is the final contract, 10 tricks being required for N-S to make with hearts as trump.

South is the declarer, having been first to bid hearts, and the player to South's left, West, has to choose the first card in the play, known as the opening lead. West chooses the spade king because spades is the suit the partnership has shown strength in, and because they have agreed that when they hold two touching honors (or adjacent honors) they will play the higher one first. West plays the card face down, to give their partner and the declarer (but not dummy) a chance to ask any last questions about the bidding or to object if they believe West is not the correct hand to lead. After that, North's cards are laid on the table and North becomes dummy, as both the North and South hands will be controlled by the declarer. West turns the lead card face up, and the declarer studies the two hands to make a plan for the play. On this hand, the trump ace, a spade, and a diamond trick must be lost, so declarer must not lose a trick in clubs.

If the K is held by West, South will find it very hard to prevent it from making a trick (unless West leads a club). There is an almost equal chance that it is held by East, in which case it can be trapped against the ace, and will be beaten, using a tactic known as a finesse.

After considering the cards, the declarer directs dummy (North) to play a small spade. East plays low (small card) and South takes the A, gaining the lead. (South may also elect to duck, but for the purpose of this example, let us assume South wins the A at trick 1). South proceeds by drawing trump, leading the K. West decides there is no benefit to holding back, and so wins the trick with the ace, and then cashes the Q. For fear of conceding a ruff and discard, West plays the 2 instead of another spade. Declarer plays low from the table, and East scores the Q. Not having anything better to do, East returns the remaining trump, taken in South's hand. The trumps now accounted for, South can now execute the finesse, perhaps trapping the king as planned. South enters the dummy (i.e. wins a trick in the dummy's hand) by leading a low diamond, using dummy's A to win the trick, and leads the Q from dummy to the next trick. East covers the queen with the king, and South takes the trick with the ace, and proceeds by cashing the remaining master J. (If East does not play the king, then South will play a low club from South's hand and the queen will win anyway, this being the essence of the finesse). The game is now safe: South ruffs a small club with a dummy's trump, then ruffs a diamond in hand for an entry back, and ruffs the last club in dummy (sometimes described as a crossruff). Finally, South claims the remaining tricks by showing his or her hand, as it now contains only high trumps and there's no need to play the hand out to prove they are all winners.

(The trick-by-trick notation used above can be also expressed in tabular form, but a textual explanation is usually preferred in practice, for reader's convenience. Plays of small cards or discards are often omitted from such a description, unless they were important for the outcome).

North-South score the required 10 tricks, and their opponents take the remaining three. The contract is fulfilled, and North enters the pair numbers, the contract, and the score of +420 for the winning side (North is in charge of bookkeeping in duplicate tournaments) on the traveling sheet. North asks East to check the score entered on the traveller. All players return their own cards to the board, and the next deal is played.

On the prior hand, it is quite possible that the K is held by West. For example, by swapping the K and A between the defending hands. Then the 4 contract would fail by one trick (unless West had led a club early in the play). The failure of the contract would not mean that 4 was a bad contract on this hand. The contract depends on the club finesse working, or a defense error. The bonus points awarded for making a game contract far outweigh the penalty for going one off, so it is best strategy in the long run to bid game contracts such as this one.

Similarly, there is a minuscule chance that the K is in the west hand, but the west hand has no other clubs. In that case, declarer can succeed by simply cashing the A, felling the K and setting up the Q as a winner. The chance of this is far lower than the chance that East started with the K. Therefore, the superior percentage play is to take the club finesse, as described above.

Computers
After many years of little progress, computer bridge made great progress at the end of the 20th century. In 1996, the ACBL initiated the official World Championships Computer Bridge, to be held annually along with a major bridge event. The first Computer Bridge Championship took place in 1997 at the North American Bridge Championships in Albuquerque, New Mexico.

Stand-alone software
Strong bridge playing programs such as Jack Bridge (World Champion in 2001, 2002, 2003, 2004, 2006, 2009, 2010, 2012, 2013 and 2015) and Wbridge5 (World Champion in 2005, 2007, 2008, 2016, 2017 and 2018), probably rank among the top few thousand human pairs worldwide. A series of articles published in 2005 and 2006 in the Dutch bridge magazine IMP describes matches between Jack Bridge and seven top Dutch pairs. A total of 196 boards were played. Jack Bridge lost, but by a small margin (359 versus 385 IMPs).

Online play
There are several free and subscription-based services available for playing bridge on the internet. For example:
 * Bridge Base Online (BBO) is the most active online bridge club in the world, with more than 100,000 daily connections and 500,000 hands played each day, in part because it is free to play regular games and volunteer-run tournaments.
 * Funbridge is a mobile and web application where users can play deals against robots. The company was started in France and is now owned by GOTO Games which is part of 52 Entertainment
 * OKbridge is the oldest extant internet bridge service: it was established as a commercial enterprise in 1994, but the program started to be used interactively in August 1990 by players of all standards. OKbridge is a subscription-based club, with services such as customer support and ethics reviews.
 * RealBridge was founded in November 2020. Its online platform includes built-in audio and video. It is primarily used for organised bridge, ranging from club level to national and zonal championships.
 * Sharkbridge founded in 2020 by Milen Milkovski (Canada), Plamen Panayotov (Canada), John Norris ( Denmark) and Michael Woywode (Germany).
 * SWAN Games was founded April 2000. In March 2004, announced a partnership to provide internet services to SBF members and is a competitor in subscription-based online bridge clubs.
 * BridgeClubLive is a subscription based club which was founded in 1994 with the Bridge Player Live Software for Windows.

Some national contract bridge organizations now offer online bridge play to their members, including the English Bridge Union, the Dutch Bridge Federation and the Australian Bridge Federation. MSN and Yahoo! Games have several online rubber bridge rooms. In 2001, the WBF issued a special edition of the lawbook adapted for internet and other electronic forms of the game.

Related card games

 * 500
 * Bridgette
 * Euchre
 * King
 * Lanterloo
 * Nap
 * Ombre
 * Quadrille
 * Rex Bridge
 * Skat
 * Spades
 * Spoil Five
 * Vint
 * Whist

title: Contract bridge


text:
Boat

A boat is a watercraft of a large range of types and sizes, but generally smaller than a ship, which is distinguished by its larger size or capacity, its shape, or its ability to carry boats.

Small boats are typically used on inland waterways such as rivers and lakes, or in protected coastal areas. However, some boats (such as whaleboats) were intended for offshore use. In modern naval terms, a boat is a vessel small enough to be carried aboard a ship.

Boats vary in proportion and construction methods with their intended purpose, available materials, or local traditions. Canoes have been used since prehistoric times and remain in use throughout the world for transportation, fishing, and sport. Fishing boats vary widely in style partly to match local conditions. Pleasure craft used in recreational boating include ski boats, pontoon boats, and sailboats. House boats may be used for vacationing or long-term residence. Lighters are used to move cargo to and from large ships unable to get close to shore. Lifeboats have rescue and safety functions.

Boats can be propelled by manpower (e.g. rowboats and paddle boats), wind (e.g. sailboats), and inboard/outboard motors (including gasoline, diesel, and electric).

Differentiation from other prehistoric watercraft
The earliest watercraft are considered to have been rafts. These would have been used for voyages such as the settlement of Australia sometime between 50,000 and 60,000 years ago.

A boat differs from a raft by obtaining its buoyancy by having most of its structure exclude water with a waterproof layer, e.g. the planks of a wooden hull, the hide covering (or tarred canvas) of a currach. In contrast, a raft is buoyant because it joins together components that are themselves buoyant, for example, logs, bamboo poles, bundles of reeds, floats (such as inflated hides, sealed pottery containers or, in a modern context, empty oil drums). The key difference between a raft and a boat is that the former is a "flow through" structure, with waves able to pass up through it. Consequently, except for short river crossings, a raft is not a practical means of transport in colder regions of the world as the users would be at risk of hypothermia. Today that climatic limitation restricts rafts to between 40° north and 40° south, with, in the past, similar boundaries that have moved as the world's climate has varied.

Types
The earliest boats may have been either dugouts or hide boats. The oldest recovered boat in the world, the Pesse canoe, found in the Netherlands, is a dugout made from the hollowed tree trunk of a Pinus sylvestris that was constructed somewhere between 8200 and 7600 BC. This canoe is exhibited in the Drents Museum in Assen, Netherlands. Other very old dugout boats have also been recovered. Hide boats, made from covering a framework with animal skins, could be equally as old as logboats, but such a structure is much less likely to survive in an archaeological context.

Plank-built boats are considered, in most cases, to have developed from the logboat. There are examples of logboats that have been expanded: by deforming the hull under the influence of heat, by raising up the sides with added planks, or by splitting down the middle and adding a central plank to make it wider. (Some of these methods have been in quite recent use – there is no simple developmental sequence). The earliest known plank-built boats are from the Nile, dating to the third millennium BC. Outside Egypt, the next earliest are from England. The Ferriby boats are dated to the early part of the second millennium BC and the end of the third millennium. Plank-built boats require a level of woodworking technology that was first available in the neolithic with more complex versions only becoming achievable in the Bronze Age.

Types

Boats can be categorized by their means of propulsion. These divide into:
 * 1) Unpowered. This involves drifting with the tide or a river current.
 * 2) Powered by the crew-members on board, using oars, paddles or a punting pole or quant.
 * 3) Powered by sail.
 * 4) Towed – either by humans or animals from a river or canal bank (or in very shallow water, by walking on the sea or river bed) or by another vessel.
 * 5) Powered by machinery, such as internal combustion engines, steam engines or by batteries and an electric motor. Any one vessel may use more than one of these methods at different times or in combination.

A number of large vessels are usually referred to as boats. Submarines are a prime example. Other types of large vessels which are traditionally called boats include Great Lakes freighters, riverboats, and ferryboats. Though large enough to carry their own boats and heavy cargo, these vessels are designed for operation on inland or protected coastal waters.

Terminology
The hull is the main, and in some cases only, structural component of a boat. It provides both capacity and buoyancy. The keel is a boat's "backbone", a lengthwise structural member to which the perpendicular frames are fixed. On some boats, a deck covers the hull, in part or whole. While a ship often has several decks, a boat is unlikely to have more than one. Above the deck are often lifelines connected to stanchions, bulwarks perhaps topped by gunnels, or some combination of the two. A cabin may protrude above the deck forward, aft, along the centerline, or cover much of the length of the boat. Vertical structures dividing the internal spaces are known as bulkheads.

The forward end of a boat is called the bow, the aft end the stern. Facing forward the right side is referred to as starboard and the left side as port.

Building materials

Until the mid-19th century, most boats were made of natural materials, primarily wood, although bark and animal skins were also used. Early boats include the birch bark canoe, the animal hide-covered kayak and coracle and the dugout canoe made from a single log.

By the mid-19th century, some boats had been built with iron or steel frames but still planked in wood. In 1855 ferro-cement boat construction was patented by the French, who coined the name "ferciment". This is a system by which a steel or iron wire framework is built in the shape of a boat's hull and covered over with cement. Reinforced with bulkheads and other internal structures it is strong but heavy, easily repaired, and, if sealed properly, will not leak or corrode.

As the forests of Britain and Europe continued to be over-harvested to supply the keels of larger wooden boats, and the Bessemer process (patented in 1855) cheapened the cost of steel, steel ships and boats began to be more common. By the 1930s boats built entirely of steel from frames to plating were seen replacing wooden boats in many industrial uses and fishing fleets. Private recreational boats of steel remain uncommon. In 1895 WH Mullins produced steel boats of galvanized iron and by 1930 became the world's largest producer of pleasure boats.

Mullins also offered boats in aluminum from 1895 through 1899 and once again in the 1920s, but it was not until the mid-20th century that aluminium gained widespread popularity. Though much more expensive than steel, aluminum alloys exist that do not corrode in salt water, allowing a similar load carrying capacity to steel at much less weight.

Around the mid-1960s, boats made of fiberglass (aka "glass fiber") became popular, especially for recreational boats. Fiberglass is also known as "GRP" (glass-reinforced plastic) in the UK, and "FRP" (for fiber-reinforced plastic) in the US. Fiberglass boats are strong and do not rust, corrode, or rot. Instead, they are susceptible to structural degradation from sunlight and extremes in temperature over their lifespan. Fiberglass structures can be made stiffer with sandwich panels, where the fiberglass encloses a lightweight core such as balsa or foam.

Cold molding is a modern construction method, using wood as the structural component. In one cold molding process, very thin strips of wood are layered over a form. Each layer is coated with resin, followed by another directionally alternating layer laid on top. Subsequent layers may be stapled or otherwise mechanically fastened to the previous, or weighted or vacuum bagged to provide compression and stabilization until the resin sets. An alternative process uses thin sheets of plywood shaped over a disposable male mold, and coated with epoxy.

Propulsion
The most common means of boat propulsion are as follows:
 * Engine
 * Inboard motor
 * Stern drive (Inboard/outboard)
 * Outboard motor
 * Paddle wheel
 * Water jet (jetboat, personal water craft)
 * Fan (hovercraft, air boat)
 * Man (rowing, paddling, setting pole etc.)
 * Wind (sailing)

Buoyancy
A boat displaces its weight in water, regardless whether it is made of wood, steel, fiberglass, or even concrete. If weight is added to the boat, the volume of the hull drawn below the waterline will increase to keep the balance above and below the surface equal. Boats have a natural or designed level of buoyancy. Exceeding it will cause the boat first to ride lower in the water, second to take on water more readily than when properly loaded, and ultimately, if overloaded by any combination of structure, cargo, and water, sink.

As commercial vessels must be correctly loaded to be safe, and as the sea becomes less buoyant in brackish areas such as the Baltic, the Plimsoll line was introduced to prevent overloading.

European Union classification
Since 1998 all new leisure boats and barges built in Europe between 2.5m and 24m must comply with the EU's Recreational Craft Directive (RCD). The Directive establishes four categories that permit the allowable wind and wave conditions for vessels in each class: Europe is the main producer of recreational boats (the second production in the world is located in Poland). European brands are known all over the world - in fact, these are the brands that created RCD and set the standard for shipyards around the world.
 * Class A - the boat may safely navigate any waters.
 * Class B - the boat is limited to offshore navigation. (Winds up to Force 8 & waves up to 4 metres)
 * Class C - the boat is limited to inshore (coastal) navigation. (Winds up to Force 6 & waves up to 2 metres)
 * Class D - the boat is limited to rivers, canals and small lakes. (Winds up to Force 4 & waves up to 0.5 metres)

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

title: Boat


text:
Blood

Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells.

Blood is composed of blood cells suspended in blood plasma. Plasma, which constitutes 55% of blood fluid, is mostly water (92% by volume), and contains proteins, glucose, mineral ions, and hormones. The blood cells are mainly red blood cells (erythrocytes), white blood cells (leukocytes), and (in mammals) platelets (thrombocytes). The most abundant cells are red blood cells. These contain hemoglobin, which facilitates oxygen transport by reversibly binding to it, increasing its solubility. Jawed vertebrates have an adaptive immune system, based largely on white blood cells. White blood cells help to resist infections and parasites. Platelets are important in the clotting of blood.

Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals with lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled. Blood is bright red when its hemoglobin is oxygenated and dark red when it is deoxygenated.

Medical terms related to blood often begin with hemo-, hemato-, haemo- or haemato- from the Greek word αἷμα (haima) for "blood". In terms of anatomy and histology, blood is considered a specialized form of connective tissue, given its origin in the bones and the presence of potential molecular fibers in the form of fibrinogen.

Functions
Blood performs many important functions within the body, including:
 * Supply of oxygen to tissues (bound to hemoglobin, which is carried in red cells)
 * Supply of nutrients such as glucose, amino acids, and fatty acids (dissolved in the blood or bound to plasma proteins (e.g., blood lipids))
 * Removal of waste such as carbon dioxide, urea, and lactic acid
 * Immunological functions, including circulation of white blood cells, and detection of foreign material by antibodies
 * Coagulation, the response to a broken blood vessel, the conversion of blood from a liquid to a semisolid gel to stop bleeding
 * Messenger functions, including the transport of hormones and the signaling of tissue damage
 * Regulation of core body temperature
 * Hydraulic functions

In mammals
Blood accounts for 7% of the human body weight, with an average density around 1060 kg/m3, very close to pure water's density of 1000 kg/m3. The average adult has a blood volume of roughly 5 L or 1.3 gallons, which is composed of plasma and formed elements. The formed elements are the two types of blood cell or corpuscle – the red blood cells, (erythrocytes) and white blood cells (leukocytes), and the cell fragments called platelets that are involved in clotting. By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, and white cells about 0.7%.

Whole blood (plasma and cells) exhibits non-Newtonian fluid dynamics.

Cells
One microliter of blood contains:
 * 4.7 to 6.1 million (male), 4.2 to 5.4 million (female) erythrocytes: Red blood cells contain the blood's hemoglobin and distribute oxygen. Mature red blood cells lack a nucleus and organelles in mammals. The red blood cells (together with endothelial vessel cells and other cells) are also marked by glycoproteins that define the different blood types. The proportion of blood occupied by red blood cells is referred to as the hematocrit, and is normally about 45%. The combined surface area of all red blood cells of the human body would be roughly 2,000 times as great as the body's exterior surface.
 * 4,000–11,000 leukocytes: White blood cells are part of the body's immune system; they destroy and remove old or aberrant cells and cellular debris, as well as attack infectious agents (pathogens) and foreign substances. The cancer of leukocytes is called leukemia.
 * 200,000–500,000 thrombocytes: Also called platelets, they take part in blood clotting (coagulation). Fibrin from the coagulation cascade creates a mesh over the platelet plug.

Plasma
About 55% of blood is blood plasma, a fluid that is the blood's liquid medium, which by itself is straw-yellow in color. The blood plasma volume totals of 2.7–3.0 liters (2.8–3.2 quarts) in an average human. It is essentially an aqueous solution containing 92% water, 8% blood plasma proteins, and trace amounts of other materials. Plasma circulates dissolved nutrients, such as glucose, amino acids, and fatty acids (dissolved in the blood or bound to plasma proteins), and removes waste products, such as carbon dioxide, urea, and lactic acid.

Other important components include:
 * Serum albumin
 * Blood-clotting factors (to facilitate coagulation)
 * Immunoglobulins (antibodies)
 * lipoprotein particles
 * Various other proteins
 * Various electrolytes (mainly sodium and chloride)

The term serum refers to plasma from which the clotting proteins have been removed. Most of the proteins remaining are albumin and immunoglobulins.

Acidity
Blood pH is regulated to stay within the narrow range of 7.35 to 7.45, making it slightly basic (compensation). Extra-cellular fluid in blood that has a pH below 7.35 is too acidic, whereas blood pH above 7.45 is too basic. A pH below 6.9 or above 7.8 is usually lethal. Blood pH, partial pressure of oxygen (pO2), partial pressure of carbon dioxide (pCO2), and bicarbonate (HCO3−) are carefully regulated by a number of homeostatic mechanisms, which exert their influence principally through the respiratory system and the urinary system to control the acid–base balance and respiration, which is called compensation. An arterial blood gas test measures these. Plasma also circulates hormones transmitting their messages to various tissues. The list of normal reference ranges for various blood electrolytes is extensive.

In non-mammals

Human blood is typical of that of mammals, although the precise details concerning cell numbers, size, protein structure, and so on, vary somewhat between species. In non-mammalian vertebrates, however, there are some key differences:
 * Red blood cells of non-mammalian vertebrates are flattened and ovoid in form, and retain their cell nuclei.
 * There is considerable variation in the types and proportions of white blood cells; for example, acidophils are generally more common than in humans.
 * Platelets are unique to mammals; in other vertebrates, small nucleated, spindle cells called thrombocytes are responsible for blood clotting instead.

Circulatory system

Blood is circulated around the body through blood vessels by the pumping action of the heart. In humans, blood is pumped from the strong left ventricle of the heart through arteries to peripheral tissues and returns to the right atrium of the heart through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins. Blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air to all of the cells of the body, and venous blood carries carbon dioxide, a waste product of metabolism by cells, to the lungs to be exhaled. However, one exception includes pulmonary arteries, which contain the most deoxygenated blood in the body, while the pulmonary veins contain oxygenated blood.

Additional return flow may be generated by the movement of skeletal muscles, which can compress veins and push blood through the valves in veins toward the right atrium.

The blood circulation was famously described by William Harvey in 1628.

Cell production and degradation
In vertebrates, the various cells of blood are made in the bone marrow in a process called hematopoiesis, which includes erythropoiesis, the production of red blood cells; and myelopoiesis, the production of white blood cells and platelets. During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs. In addition, during childhood, the thymus gland, found in the mediastinum, is an important source of T lymphocytes. The proteinaceous component of blood (including clotting proteins) is produced predominantly by the liver, while hormones are produced by the endocrine glands and the watery fraction is regulated by the hypothalamus and maintained by the kidney.

Healthy erythrocytes have a plasma life of about 120 days before they are degraded by the spleen, and the Kupffer cells in the liver. The liver also clears some proteins, lipids, and amino acids. The kidney actively secretes waste products into the urine.

Oxygen transport
About 98.5% of the oxygen in a sample of arterial blood in a healthy human breathing air at sea-level pressure is chemically combined with the hemoglobin. About 1.5% is physically dissolved in the other blood liquids and not connected to hemoglobin. The hemoglobin molecule is the primary transporter of oxygen in mammals and many other species. Hemoglobin has an oxygen binding capacity between 1.36 and 1.40 ml O2 per gram hemoglobin, which increases the total blood oxygen capacity seventyfold, compared to if oxygen solely were carried by its solubility of 0.03 ml O2 per liter blood per mm Hg partial pressure of oxygen (about 100 mm Hg in arteries).

With the exception of pulmonary and umbilical arteries and their corresponding veins, arteries carry oxygenated blood away from the heart and deliver it to the body via arterioles and capillaries, where the oxygen is consumed; afterwards, venules and veins carry deoxygenated blood back to the heart.

Under normal conditions in adult humans at rest, hemoglobin in blood leaving the lungs is about 98–99% saturated with oxygen, achieving an oxygen delivery between 950 and 1150 ml/min to the body. In a healthy adult at rest, oxygen consumption is approximately 200–250 ml/min, and deoxygenated blood returning to the lungs is still roughly 75% (70 to 78%) saturated. Increased oxygen consumption during sustained exercise reduces the oxygen saturation of venous blood, which can reach less than 15% in a trained athlete; although breathing rate and blood flow increase to compensate, oxygen saturation in arterial blood can drop to 95% or less under these conditions. Oxygen saturation this low is considered dangerous in an individual at rest (for instance, during surgery under anesthesia). Sustained hypoxia (oxygenation less than 90%), is dangerous to health, and severe hypoxia (saturations less than 30%) may be rapidly fatal.

A fetus, receiving oxygen via the placenta, is exposed to much lower oxygen pressures (about 21% of the level found in an adult's lungs), so fetuses produce another form of hemoglobin with a much higher affinity for oxygen (hemoglobin F) to function under these conditions.

Carbon dioxide transport
CO2 is carried in blood in three different ways. (The exact percentages vary depending whether it is arterial or venous blood). Most of it (about 70%) is converted to bicarbonate ions HCO3− by the enzyme carbonic anhydrase in the red blood cells by the reaction CO2 + H2O -> H2CO3 -> H+ + HCO3−; about 7% is dissolved in the plasma; and about 23% is bound to hemoglobin as carbamino compounds.

Hemoglobin, the main oxygen-carrying molecule in red blood cells, carries both oxygen and carbon dioxide. However, the CO2 bound to hemoglobin does not bind to the same site as oxygen. Instead, it combines with the N-terminal groups on the four globin chains. However, because of allosteric effects on the hemoglobin molecule, the binding of CO2 decreases the amount of oxygen that is bound for a given partial pressure of oxygen. The decreased binding to carbon dioxide in the blood due to increased oxygen levels is known as the Haldane effect, and is important in the transport of carbon dioxide from the tissues to the lungs. A rise in the partial pressure of CO2 or a lower pH will cause offloading of oxygen from hemoglobin, which is known as the Bohr effect.

Transport of hydrogen ions
Some oxyhemoglobin loses oxygen and becomes deoxyhemoglobin. Deoxyhemoglobin binds most of the hydrogen ions as it has a much greater affinity for more hydrogen than does oxyhemoglobin.

Lymphatic system
In mammals, blood is in equilibrium with lymph, which is continuously formed in tissues from blood by capillary ultrafiltration. Lymph is collected by a system of small lymphatic vessels and directed to the thoracic duct, which drains into the left subclavian vein, where lymph rejoins the systemic blood circulation.

Thermoregulation
Blood circulation transports heat throughout the body, and adjustments to this flow are an important part of thermoregulation. Increasing blood flow to the surface (e.g., during warm weather or strenuous exercise) causes warmer skin, resulting in faster heat loss. In contrast, when the external temperature is low, blood flow to the extremities and surface of the skin is reduced and to prevent heat loss and is circulated to the important organs of the body, preferentially.

Rate of flow
Rate of blood flow varies greatly between different organs. Liver has the most abundant blood supply with an approximate flow of 1350 ml/min. Kidney and brain are the second and the third most supplied organs, with 1100 ml/min and ~700 ml/min, respectively.

Relative rates of blood flow per 100 g of tissue are different, with kidney, adrenal gland and thyroid being the first, second and third most supplied tissues, respectively.

Hydraulic functions
The restriction of blood flow can also be used in specialized tissues to cause engorgement, resulting in an erection of that tissue; examples are the erectile tissue in the penis and clitoris.

Another example of a hydraulic function is the jumping spider, in which blood forced into the legs under pressure causes them to straighten for a powerful jump, without the need for bulky muscular legs.

Color
Hemoglobin is the principal determinant of the color of blood (hemochrome). Each molecule has four heme groups, and their interaction with various molecules alters the exact color. Arterial blood and capillary blood are bright red, as oxygen imparts a strong red color to the heme group. Deoxygenated blood is a darker shade of red; this is present in veins, and can be seen during blood donation and when venous blood samples are taken. This is because the spectrum of light absorbed by hemoglobin differs between the oxygenated and deoxygenated states.

Blood in carbon monoxide poisoning is bright red, because carbon monoxide causes the formation of carboxyhemoglobin. In cyanide poisoning, the body cannot use oxygen, so the venous blood remains oxygenated, increasing the redness. There are some conditions affecting the heme groups present in hemoglobin that can make the skin appear blue – a symptom called cyanosis. If the heme is oxidized, methemoglobin, which is more brownish and cannot transport oxygen, is formed. In the rare condition sulfhemoglobinemia, arterial hemoglobin is partially oxygenated, and appears dark red with a bluish hue.

Veins close to the surface of the skin appear blue for a variety of reasons. However, the factors that contribute to this alteration of color perception are related to the light-scattering properties of the skin and the processing of visual input by the visual cortex, rather than the actual color of the venous blood.

Skinks in the genus Prasinohaema have green blood due to a buildup of the waste product biliverdin.

General medical

 * Disorders of volume
 * Injury can cause blood loss through bleeding. A healthy adult can lose almost 20% of blood volume (1 L) before the first symptom, restlessness, begins, and 40% of volume (2 L) before shock sets in. Thrombocytes are important for blood coagulation and the formation of blood clots, which can stop bleeding. Trauma to the internal organs or bones can cause internal bleeding, which can sometimes be severe.
 * Dehydration can reduce the blood volume by reducing the water content of the blood. This would rarely result in shock (apart from the very severe cases) but may result in orthostatic hypotension and fainting.
 * Disorders of circulation
 * Shock is the ineffective perfusion of tissues, and can be caused by a variety of conditions including blood loss, infection, poor cardiac output.
 * Atherosclerosis reduces the flow of blood through arteries, because atheroma lines arteries and narrows them. Atheroma tends to increase with age, and its progression can be compounded by many causes including smoking, high blood pressure, excess circulating lipids (hyperlipidemia), and diabetes mellitus.
 * Coagulation can form a thrombosis, which can obstruct vessels.
 * Problems with blood composition, the pumping action of the heart, or narrowing of blood vessels can have many consequences including hypoxia (lack of oxygen) of the tissues supplied. The term ischemia refers to tissue that is inadequately perfused with blood, and infarction refers to tissue death (necrosis), which can occur when the blood supply has been blocked (or is very inadequate).

Hematological

 * Anemia
 * Insufficient red cell mass (anemia) can be the result of bleeding, blood disorders like thalassemia, or nutritional deficiencies, and may require one or more blood transfusions. Anemia can also be due to a genetic disorder in which the red blood cells do not function effectively. Anemia can be confirmed by a blood test if the hemoglobin value is less than 13.5 gm/dl in men or less than 12.0 gm/dl in women. Several countries have blood banks to fill the demand for transfusable blood. A person receiving a blood transfusion must have a blood type compatible with that of the donor.
 * Sickle-cell anemia
 * Disorders of cell proliferation
 * Leukemia is a group of cancers of the blood-forming tissues and cells.
 * Non-cancerous overproduction of red cells (polycythemia vera) or platelets (essential thrombocytosis) may be premalignant.
 * Myelodysplastic syndromes involve ineffective production of one or more cell lines.
 * Disorders of coagulation
 * Hemophilia is a genetic illness that causes dysfunction in one of the blood's clotting mechanisms. This can allow otherwise inconsequential wounds to be life-threatening, but more commonly results in hemarthrosis, or bleeding into joint spaces, which can be crippling.
 * Ineffective or insufficient platelets can also result in coagulopathy (bleeding disorders).
 * Hypercoagulable state (thrombophilia) results from defects in regulation of platelet or clotting factor function, and can cause thrombosis.
 * Infectious disorders of blood
 * Blood is an important vector of infection. HIV, the virus that causes AIDS, is transmitted through contact with blood, semen or other body secretions of an infected person. Hepatitis B and C are transmitted primarily through blood contact. Owing to blood-borne infections, bloodstained objects are treated as a biohazard.
 * Bacterial infection of the blood is bacteremia or sepsis. Viral Infection is viremia. Malaria and trypanosomiasis are blood-borne parasitic infections.

Carbon monoxide poisoning
Substances other than oxygen can bind to hemoglobin; in some cases, this can cause irreversible damage to the body. Carbon monoxide, for example, is extremely dangerous when carried to the blood via the lungs by inhalation, because carbon monoxide irreversibly binds to hemoglobin to form carboxyhemoglobin, so that less hemoglobin is free to bind oxygen, and fewer oxygen molecules can be transported throughout the blood. This can cause suffocation insidiously. A fire burning in an enclosed room with poor ventilation presents a very dangerous hazard, since it can create a build-up of carbon monoxide in the air. Some carbon monoxide binds to hemoglobin when smoking tobacco.

Transfusion
Blood for transfusion is obtained from human donors by blood donation and stored in a blood bank. There are many different blood types in humans, the ABO blood group system, and the Rhesus blood group system being the most important. Transfusion of blood of an incompatible blood group may cause severe, often fatal, complications, so crossmatching is done to ensure that a compatible blood product is transfused.

Other blood products administered intravenously are platelets, blood plasma, cryoprecipitate, and specific coagulation factor concentrates.

Intravenous administration
Many forms of medication (from antibiotics to chemotherapy) are administered intravenously, as they are not readily or adequately absorbed by the digestive tract.

After severe acute blood loss, liquid preparations, generically known as plasma expanders, can be given intravenously, either solutions of salts (NaCl, KCl, CaCl2 etc.) at physiological concentrations, or colloidal solutions, such as dextrans, human serum albumin, or fresh frozen plasma. In these emergency situations, a plasma expander is a more effective life-saving procedure than a blood transfusion, because the metabolism of transfused red blood cells does not restart immediately after a transfusion.

Letting
In modern evidence-based medicine, bloodletting is used in management of a few rare diseases, including hemochromatosis and polycythemia. However, bloodletting and leeching were common unvalidated interventions used until the 19th century, as many diseases were incorrectly thought to be due to an excess of blood, according to Hippocratic medicine.

Etymology
English blood (Old English blod) derives from Germanic and has cognates with a similar range of meanings in all other Germanic languages (e.g. German Blut, Swedish blod, Gothic blōþ). There is no accepted Indo-European etymology.

Classical Greek medicine
Robin Fåhræus (a Swedish physician who devised the erythrocyte sedimentation rate) suggested that the Ancient Greek system of humorism, wherein the body was thought to contain four distinct bodily fluids (associated with different temperaments), were based upon the observation of blood clotting in a transparent container. When blood is drawn in a glass container and left undisturbed for about an hour, four different layers can be seen. A dark clot forms at the bottom (the "black bile"). Above the clot is a layer of red blood cells (the "blood"). Above this is a whitish layer of white blood cells (the "phlegm"). The top layer is clear yellow serum (the "yellow bile").

Types
The ABO blood group system was discovered in the year 1900 by Karl Landsteiner. Jan Janský is credited with the first classification of blood into the four types (A, B, AB, and O) in 1907, which remains in use today. In 1907 the first blood transfusion was performed that used the ABO system to predict compatibility. The first non-direct transfusion was performed on 27 March 1914. The Rhesus factor was discovered in 1937.

Culture and religion
Due to its importance to life, blood is associated with a large number of beliefs. One of the most basic is the use of blood as a symbol for family relationships through birth/parentage; to be "related by blood" is to be related by ancestry or descendence, rather than marriage. This bears closely to bloodlines, and sayings such as "blood is thicker than water" and "bad blood", as well as "Blood brother".

Blood is given particular emphasis in the Islamic, Jewish, and Christian religions, because Leviticus 17:11 says "the life of a creature is in the blood." This phrase is part of the Levitical law forbidding the drinking of blood or eating meat with the blood still intact instead of being poured off.

Mythic references to blood can sometimes be connected to the life-giving nature of blood, seen in such events as childbirth, as contrasted with the blood of injury or death.

Indigenous Australians
In many indigenous Australian Aboriginal peoples' traditions, ochre (particularly red) and blood, both high in iron content and considered Maban, are applied to the bodies of dancers for ritual. As Lawlor states:"In many Aboriginal rituals and ceremonies, red ochre is rubbed all over the naked bodies of the dancers. In secret, sacred male ceremonies, blood extracted from the veins of the participant's arms is exchanged and rubbed on their bodies. Red ochre is used in similar ways in less-secret ceremonies. Blood is also used to fasten the feathers of birds onto people's bodies. Bird feathers contain a protein that is highly magnetically sensitive." Lawlor comments that blood employed in this fashion is held by these peoples to attune the dancers to the invisible energetic realm of the Dreamtime. Lawlor then connects these invisible energetic realms and magnetic fields, because iron is magnetic.

European paganism
Among the Germanic tribes, blood was used during their sacrifices; the Blóts. The blood was considered to have the power of its originator, and, after the butchering, the blood was sprinkled on the walls, on the statues of the gods, and on the participants themselves. This act of sprinkling blood was called blóedsian in Old English, and the terminology was borrowed by the Roman Catholic Church becoming to bless and blessing. The Hittite word for blood, ishar was a cognate to words for "oath" and "bond", see Ishara. The Ancient Greeks believed that the blood of the gods, ichor, was a substance that was poisonous to mortals.

As a relic of Germanic Law, the cruentation, an ordeal where the corpse of the victim was supposed to start bleeding in the presence of the murderer, was used until the early 17th century.

Christianity
In Genesis 9:4, God prohibited Noah and his sons from eating blood (see Noahide Law). This command continued to be observed by the Eastern Orthodox Church.

It is also found in the Bible that when the Angel of Death came around to the Hebrew house that the first-born child would not die if the angel saw lamb's blood wiped across the doorway.

At the Council of Jerusalem, the apostles prohibited certain Christians from consuming blood – this is documented in Acts 15:20 and 29. This chapter specifies a reason (especially in verses 19–21): It was to avoid offending Jews who had become Christians, because the Mosaic Law Code prohibited the practice.

Christ's blood is the means for the atonement of sins. Also, "... the blood of Jesus Christ his [God] Son cleanseth us from all sin." (1 John 1:7), "... Unto him [God] that loved us, and washed us from our sins in his own blood." (Revelation 1:5), and "And they overcame him (Satan) by the blood of the Lamb [Jesus the Christ], and by the word of their testimony ..." (Revelation 12:11).

Some Christian churches, including Roman Catholicism, Eastern Orthodoxy, Oriental Orthodoxy, and the Assyrian Church of the East teach that, when consecrated, the Eucharistic wine actually becomes the blood of Jesus for worshippers to drink. Thus in the consecrated wine, Jesus becomes spiritually and physically present. This teaching is rooted in the Last Supper, as written in the four gospels of the Bible, in which Jesus stated to his disciples that the bread that they ate was his body, and the wine was his blood. ''"This cup is the new testament in my blood, which is shed for you." ''.

Most forms of Protestantism, especially those of a Methodist or Presbyterian lineage, teach that the wine is no more than a symbol of the blood of Christ, who is spiritually but not physically present. Lutheran theology teaches that the body and blood is present together "in, with, and under" the bread and wine of the Eucharistic feast.

Judaism
In Judaism, animal blood may not be consumed even in the smallest quantity (Leviticus 3:17 and elsewhere); this is reflected in Jewish dietary laws (Kashrut). Blood is purged from meat by rinsing and soaking in water (to loosen clots), salting and then rinsing with water again several times. Eggs must also be checked and any blood spots removed before consumption. Although blood from fish is biblically kosher, it is rabbinically forbidden to consume fish blood to avoid the appearance of breaking the Biblical prohibition.

Another ritual involving blood involves the covering of the blood of fowl and game after slaughtering (Leviticus 17:13); the reason given by the Torah is: "Because the life of the animal is [in] its blood" (ibid 17:14). In relation to human beings, Kabbalah expounds on this verse that the animal soul of a person is in the blood, and that physical desires stem from it.

Likewise, the mystical reason for salting temple sacrifices and slaughtered meat is to remove the blood of animal-like passions from the person. By removing the animal's blood, the animal energies and life-force contained in the blood are removed, making the meat fit for human consumption.

Islam
Consumption of food containing blood is forbidden by Islamic dietary laws. This is derived from the statement in the Qur'an, sura Al-Ma'ida (5:3): "Forbidden to you (for food) are: dead meat, blood, the flesh of swine, and that on which has been invoked the name of other than Allah."

Blood is considered unclean, hence there are specific methods to obtain physical and ritual status of cleanliness once bleeding has occurred. Specific rules and prohibitions apply to menstruation, postnatal bleeding and irregular vaginal bleeding. When an animal has been slaughtered, the animal's neck is cut in a way to ensure that the spine is not severed, hence the brain may send commands to the heart to pump blood to it for oxygen. In this way, blood is removed from the body, and the meat is generally now safe to cook and eat. In modern times, blood transfusions are generally not considered against the rules.

Jehovah's Witnesses
Based on their interpretation of scriptures such as Acts 15:28, 29 ("Keep abstaining...from blood."), many Jehovah's Witnesses neither consume blood nor accept transfusions of whole blood or its major components: red blood cells, white blood cells, platelets (thrombocytes), and plasma. Members may personally decide whether they will accept medical procedures that involve their own blood or substances that are further fractionated from the four major components.

Vampirism
Vampires are mythical creatures that drink blood directly for sustenance, usually with a preference for human blood. Cultures all over the world have myths of this kind; for example the 'Nosferatu' legend, a human who achieves damnation and immortality by drinking the blood of others, originates from Eastern European folklore. Ticks, leeches, female mosquitoes, vampire bats, and an assortment of other natural creatures do consume the blood of other animals, but only bats are associated with vampires. This has no relation to vampire bats, which are New World creatures discovered well after the origins of the European myths.

Invertebrates
In invertebrates, a body fluid analogous to blood called hemolymph is found, the main difference being that hemolymph is not contained in a closed circulatory system. Hemolymph may function to carry oxygen, although hemoglobin is not necessarily used. Crustaceans and mollusks use hemocyanin instead of hemoglobin. In most insects, their hemolymph does not contain oxygen-carrying molecules because their bodies are small enough for their tracheal system to suffice for supplying oxygen.

Forensic and archaeological
Blood residue can help forensic investigators identify weapons, reconstruct a criminal action, and link suspects to the crime. Through bloodstain pattern analysis, forensic information can also be gained from the spatial distribution of bloodstains.

Blood residue analysis is also a technique used in archeology.

Artistic
Blood is one of the body fluids that has been used in art. In particular, the performances of Viennese Actionist Hermann Nitsch, Istvan Kantor, Franko B, Lennie Lee, Ron Athey, Yang Zhichao, Lucas Abela and Kira O'Reilly, along with the photography of Andres Serrano, have incorporated blood as a prominent visual element. Marc Quinn has made sculptures using frozen blood, including a cast of his own head made using his own blood.

Genealogical
The term blood is used in genealogical circles to refer to one's ancestry, origins, and ethnic background as in the word bloodline. Other terms where blood is used in a family history sense are blue-blood, royal blood, mixed-blood and blood relative.

title: Blood