Go

Go is played on a grid of black lines (usually 19×19). The playing pieces, called stones, are played on the intersections of the lines.
Years active	Zhou Dynasty to present
Genre(s)	Board game
Players	2
Age range	3+[1][nb 1]
Setup time	Minimal
Playing time	Casual: 20–90 minutes Tournament: 1–6 hours[a]
Random chance	None
Skill(s) required	Tactics, strategy, observation
a Some professional games exceed 16 hours and are played in sessions spread over two days.

Go (圍棋 weiqi in Chinese, 囲碁 igo in Japanese, 바둑 baduk in Korean, common meaning: "encircling game") is a board game for two players that originated in China more than 2,000 years ago. The game is noted for being rich in strategy despite its relatively simple rules.

The two players alternately place black and white markers, called "stones", on the vacant intersections (called "points") of a grid of 19×19 lines (beginners often play on smaller 9×9 and 13×13 boards).^[2] The object of the game is to use one's stones to surround a larger portion of the board than the opponent.^[3] Once placed on the board, stones can only be moved if they are captured (removed from board).^[4] When a game concludes, the controlled points (territory) are counted along with captured stones to determine who has more points.^[5] Games may also be won by resignation.

Go originated in ancient China. Archaeological evidence shows that the early game was played on a board with a 17×17 grid, but by the time the game had spread to Korea and Japan, in about the 5th and 7th centuries CE respectively, boards with a 19×19 grid had become standard.

Although the overwhelming majority of players are from East Asia, the International Go Federation has 71 member countries on all continents except Antarctica. The total number of players is well over 40 million.^[6] Go reached the West through Japan, which is why it is commonly known internationally by its shortened Japanese name, and why terms for common Go concepts are commonly derived from their Japanese pronunciation. The full Japanese name igo is derived from its Chinese name weiqi ("way-chee"), which roughly translates as "board game of surrounding".^{[nb 2]}

Overview[link]

The first 60 moves of a Go game, animated. This particular game quickly developed into a complicated fight in the lower left and bottom.

Go is an adversarial game with the objective of surrounding more territory than one's opponent.^[7] As the game progresses, the board gets divided up into areas of territory, as outlined by groups of stones. These areas are then contested in local battles, which are often complicated, and may result in the expansion, reduction, or wholesale capture and loss of the contested area.

The four liberties (adjacent empty points) of a single black stone (A), as white reduces those liberties by one (B, C, and D). When black has only one liberty left (D), that stone is "in atari".^[8] white may capture that stone (remove from board) with a play on its last liberty (at D-1).

The basic principle of Go is that stones must have "liberty" (Chinese: 氣) to remain on the board. A liberty is an open point (intersection) next to a stone.^[9] An enclosed liberty (or liberties) is called an "eye" (眼), and a group of stones with at least two eyes is said to be unconditionally "alive". Such groups cannot be captured, even if surrounded.^[10] "Dead" stones are those which are surrounded and in groups with poor shape (one or no eyes) and thus cannot resist eventual capture.

The liberties of groups are countable. In cases where two opposing groups must capture the other to live, these "capturing races" ("semeai") reduce to a logical contest of relative liberties—the group with more liberties (and/or "better shape") captures the opponent's stones and thus lives.^[11] Semeais and life and death problems are examples of where parts of the game can form a type of puzzle.

The general strategy of Go is to expand one's territory where possible, attack the opponent's weak groups (groups that can possibly be killed), and always stay mindful of the "life status" of one's own groups.^[12] The game ends when both players pass, and players pass when there are no more profitable moves to be made. Games may also be won by resignation, for example if a player has lost a large group of stones.

Details[link]

The game's opening stages largely deal with establishing "bases" (or "footholds") in the corners and around the sides of the board. These bases help to quickly establish living shapes and surround sections of territory. The first principle of opening theory is to play in the corners, because it is more efficient there to make life and to establish territory.^[13] Corner opening sequences are called "joseki" and are studied independently as parts of the game.

Playing with others usually requires a knowledge of each player's strength, as indicated by a rank. Handicaps are given if there is a notable difference in rank — black is allowed to place two or more stones on the board to compensate for white's greater strength.^[14] There are different rule-sets (Japanese, Chinese, AGA, etc.), which are almost entirely equivalent, except for certain special-case positions.

"Dame" ("dah-may") are points that lie in-between the boundary walls of black and white, and as such are considered to be of no value to either side. "Sekis" (Chinese: 共活) are mutually alive pairs of white and black groups where neither has two eyes. A "ko" (Chinese and Japanese: 劫) is a repeated-position shape that may be contested by making forcing moves elsewhere. After the forcing move is played, the ko may be "taken back" and returned to its original position. Some "ko fights" may be important and decide the life of a large group, while others may be worth just one or two points.

Rules[link]

Although there are some minor differences between rule sets used in different countries,^[15] most notably in Chinese and Japanese scoring rules,^[16] these differences do not seriously affect the tactics and strategy of the game. Except where noted otherwise, the basic rules presented here are valid independent of the scoring rules used. The scoring rules are explained separately. Go concepts for which there are no ready English equivalent are commonly called by their Japanese names.

Basic rules[link]

One black chain and two white chains, with their liberties marked with dots. Liberties are shared among all stones of a chain and can be counted. Here the black group has 5 liberties, while the two white chains have 4 liberties each.

Two players, black and white, take turns placing a stone (game piece) of their own color on a vacant point (intersection) of the grid on a Go board. Black moves first. If there is a large difference in skill between the players, black is typically allowed to place two or more stones on the board to compensate for the difference (see Go handicaps). The official grid comprises 19×19 lines, though the rules can be applied to any grid size. 13×13 and 9×9 boards are popular choices to teach beginners.^[17] Once placed, a stone may not be moved to a different point.^[18]

Vertically and horizontally adjacent stones of the same color form a chain (also called a string, or simply a group) that shares its liberties (see below) in common, cannot subsequently be subdivided, and in effect becomes a single larger stone.^[19] Only stones connected to one another by the lines on the board create a chain; stones that are diagonally adjacent are not connected. Chains may be expanded by placing additional stones on adjacent intersections, and can be connected together by placing a stone on an intersection that is adjacent to two or more chains of the same color.

If white plays at A, the black chain loses its last liberty. It is captured and removed from the board. Note the resulting shape for White is good, meaning highly resistant to capture.

A vacant point adjacent to a stone is called a liberty for that stone.^{[20][nb 3]} Stones in a chain share their liberties. A chain of stones must have at least one liberty to remain on the board. When a chain is surrounded by opposing stones so that it has no liberties, it is captured and removed from the board.

The ko rule[link]

An example of a situation in which the ko rule applies

Players are not allowed to make a move that returns the game to the previous position. This rule, called the ko rule (Chinese: 劫; Japanese: 劫 kō "eon"), prevents unending repetition.^[21] See the example to the right: Black has just played the stone marked 1, capturing a white stone at the intersection marked with a circle. If white were now allowed to play on the marked intersection, that move would capture the black stone marked 1 and recreate the situation before black made the move marked 1. Allowing this could result in an unending cycle of captures by both players. The ko rule therefore prohibits White from playing at the marked intersection immediately. Instead white must play elsewhere; black can then end the ko by filling at the marked intersection, creating a five-stone black chain. If White wants to continue the ko (that specific repeating position), white will try to find a play elsewhere on the board that black must answer; if black answers, then white can retake the ko. A repetition of such exchanges is called a ko fight.^[22]

While the various rule sets agree on the ko rule prohibiting returning the board to an immediately previous position, they deal in different ways with the relatively uncommon situation in which a player might recreate a past position that is further removed. See Rules of Go: Repetition for further information.

Passing[link]

Instead of placing a stone, a player may pass. This usually occurs when they believe no useful moves remain. When both players pass consecutively, the game ends and is then scored.

Playing stones with no liberties[link]

Under normal rules, white cannot play at A because that point has no liberties, hence the black group is alive. Under the Ing and New Zealand rulesets, white may play A as a ko threat, leaving an empty three-space eye. Black will naturally answer by playing at A, creating two eyes.

A player may not place a stone such that it or its group immediately has no liberties, unless doing so immediately deprives an enemy group of its final liberty. In the latter case, the enemy group is captured, leaving the new stone with at least one liberty.^[23] This rule is responsible for the all-important difference between one and two eyes: if a group with only one eye is fully surrounded on the outside, it can be killed with a stone placed in its single eye.

The Ing rules and New Zealand rules allow an exception to this rule, such that a player might destroy one of its own groups—"commit suicide". This play would only be useful in a limited set of situations involving a small interior space.^[24]

Komi[link]

Because black has a first move advantage, the idea of awarding White some compensation came into being during the 20th century. This predetermined compensation, called "komidashi", ("komi") means that white may receive somewhere on average of 6.5 points compensation for black's first move advantage. If there is one stone difference in strength between players, the stronger player takes white, and white may receive only .5 points komi, to break a possible tie ("jigo"). In handicap games with 2 or more handicap stones, White may also take .5 points komi to break a tie.

Scoring rules[link]

A simplified game at its end. Black's territory (A) is counted and compared to White's territory (B). In this example, both black and white attempted to invade and live (C and D groups) to reduce the other's total territory. Only black's invading group (C) was successful in living, as white's group (D) was killed with a black stone at E. The points in the middle (F) are "dame", meaning they belong to neither player.

Two general types of scoring system are used, and players determine which to use before play. Both systems almost always give the same result. Territory scoring counts the number of empty points a player's stones surround, together with the number of stones he captured. While it originated in China, today it is commonly associated with Japan and Korea. Area scoring counts the number of points your stones occupy and surround. It is associated with contemporary Chinese play and was probably established there during the Ming Dynasty in the 15th or 16th century.^[25]

Detailed description[link]

After both players have passed consecutively, the stones that are still on the board but unable to avoid capture, called dead stones, are removed.

Area scoring (including Chinese): A player's score is the number of stones he has on the board, plus the number of empty intersections surrounded by that player's stones.

Territory scoring (including Japanese and Korean): In the course of the game, each player retains the stones they capture, termed prisoners. Any dead stones removed at the end of the game become prisoners. The score is the number of empty points enclosed by a player's stones, plus the number of prisoners captured by that player.^{[nb 4]}

If there is disagreement about which stones are dead, then under area scoring rules, the players simply resume play to resolve the matter. The score is computed using the position after the next time the players pass consecutively. Under territory scoring, the rules are considerably more complex; however, in practice, players generally play on, and, once the status of each stone has been determined, return to the position at the time the first two consecutive passes occurred and remove the dead stones. For further information, see Rules of Go.

Given that the number of stones a player has on the board is related to the number of prisoners the opponent has taken, the resulting net score (the difference between Black and White's respective scores) under both rulesets is often identical and rarely differs by more than a point.^[26]

Life and death[link]

While not actually mentioned in the rules of Go (at least in simpler rule sets, such as those of New Zealand and the US), the concept of a living group of stones is necessary for a practical understanding of the game.^[27]

Examples of eyes (marked). The black groups at the top of the board are alive, as they have at least two eyes. The black groups at the bottom are dead as they only have one eye.

When a group of stones is mostly surrounded and has no options to connect with friendly stones elsewhere, the status of the group is either alive, dead or unsettled. A group of stones is said to be alive if it cannot be captured, even if the opponent is allowed to move first. Conversely, a group of stones is said to be dead if it cannot avoid capture, even if the owner of the group is allowed the first move. Otherwise, the group is said to be unsettled: in such a situation, the player that moves first may be able to either make it alive if he is the owner, or kill it if he is the group owner's opponent.^[27]

To be alive, a group must be able to create at least two "eyes" if threatened. An eye is an empty point that is surrounded by friendly stones, where the opponent can never play due to the suicide rule. If two such eyes exist, the opponent can never capture a group of stones, because it always has at least two liberties. One eye is not enough for life, because a point that would normally be suicide may be played upon if doing so fills the last liberty of opposing stones, thereby capturing those stones. In the "Examples of eyes" diagram, all the circled points are eyes. The two black groups in the upper corners are alive, as both have at least two eyes. The groups in the lower corners are dead, as both have only one eye. The group in the lower left may seem to have two eyes, but the surrounded empty point without a circle is not actually an eye. White can play there and take a black stone. Such a point is often called a false eye.^[27]

Seki (mutual life)[link]

Example of seki (mutual life). Neither black nor white can play on the marked points without reducing their own liberties for those groups to one (self-atari).

There is an exception to the requirement that a group must have two eyes to be alive, a situation called seki (or mutual life). Where different coloured groups are adjacent and share liberties, the situation may reach a position when neither player wants to move first, because doing so would allow the opponent to capture; in such situations therefore both player's stones remain on the board in mutual life or "seki". Neither player receives any points for those groups, but at least those groups themselves remain living, as opposed to being captured.^{[nb 5]}

Sekis can occur in many ways. The simplest are: (1) each player has a group without eyes and they share two liberties, and (2) each player has a group with one eye and they share one more liberty. In the "Example of seki (mutual life)" diagram, the circled points are liberties shared by both a black and a white group. Neither player wants to play on a circled point, because doing so would allow the opponent to capture. All the other groups in this example, both black and white, are alive with at least two eyes. Sekis are unusual, but can result from an attempt by one player to invade and kill a nearly settled group of the other player.^[27]

Tactics[link]

In Go, tactics deal with immediate fighting between stones, capturing and saving stones, life, death and other issues localized to a specific part of the board. Larger issues, not limited to only part of the board, are referred to as strategy, and are covered in their own section.

Capturing tactics[link]

There are several tactical constructs aimed at capturing stones.^[28] These are among the first things a player learns after understanding the rules. Recognizing the possibility that stones can be captured using these techniques is an important step forward.

A ladder. Black cannot escape unless the ladder connects to friendly stones further down the board.

The most basic technique is the ladder.^[29] To capture stones in a ladder, a player uses a constant series of capture threats—called atari—to force the opponent into a zigzag pattern as shown in the diagram to the right. Unless the pattern runs into friendly stones along the way, the stones in the ladder cannot avoid capture. Experienced players recognize the futility of continuing the pattern and play elsewhere. The presence of a ladder on the board does give a player the option to play a stone in the path of the ladder, thereby threatening to rescue their stones, forcing a response. Such a move is called a ladder breaker and may be a powerful strategic move. In the diagram, black has the option of playing a ladder breaker.

A net. The chain of three marked black stones cannot escape in any direction.

Another technique to capture stones is the so-called net,^[30] also known by its Japanese name, geta. This refers to a move that loosely surrounds some stones, preventing their escape in all directions. An example is given in the diagram to the left. It is generally better to capture stones in a net than in a ladder, because a net does not depend on the condition that there are no opposing stones in the way, nor does it allow the opponent to play a strategic ladder breaker.

A snapback. Although black can capture the white stone by playing at the circled point, the resulting shape for black has only one liberty (at 1), thus White can then capture the three black stones by playing at 1 again (snap back).

A third technique to capture stones is the snapback.^[31] In a snapback, one player allows a single stone to be captured, then immediately plays on the point formerly occupied by that stone; by so doing, the player captures a larger group of their opponent's stones, in effect snapping back at those stones. An example can be seen on the right. As with the ladder, an experienced player does not play out such a sequence, recognizing the futility of capturing only to be captured back immediately.

Reading ahead[link]

One of the most important skills required for strong tactical play is the ability to read ahead. Reading ahead includes considering available moves to play, the possible responses to each move, and the subsequent possibilities after each of those responses. Some of the strongest players of the game can read up to 40 moves ahead even in complicated positions.^[32]

As explained in the scoring rules, some stone formations can never be captured and are said to be alive, while other stones may be in the position where they cannot avoid being captured and are said to be dead.Much of the practice material available to students of the game comes in the form of life and death problems, also known as tsumego.^[33] In such problems, players are challenged to find the vital move sequence that will kill a group of the opponent or save a group of their own. Tsumego are considered an excellent way to train a player's ability at reading ahead,^[33] and are available for all skill levels, some posing a challenge even to top players.

Ko fighting[link]

A simplified ko fight on a 9×9 board. The ko is at the point marked with a square—black has "taken the ko" first. The ko fight determines the life of the A and B groups—only one survives and the other is captured. White may play C as a ko threat, and black properly answers at D. White can then "take the ko" by playing at the square-marked point (capturing the one black stone). E is a possible ko threat for black.

In situations when the Ko rule applies, a ko fight may occur.^[22] If the player who is prohibited from capture is of the opinion that the capture is important, because it prevents a large group of stones from being captured for instance, the player may play a ko threat.^[22] This is a move elsewhere on the board that threatens to make a large profit if the opponent does not respond. If the opponent does respond to the ko threat, the situation on the board has changed, and the prohibition on capturing the ko no longer applies. Thus the player who made the ko threat may now recapture the ko. Their opponent is then in the same situation and can either play a ko threat as well, or concede the ko by simply playing elsewhere. If a player concedes the ko, either because they do not think it important or because there are no moves left that could function as a ko threat, they have lost the ko, and their opponent may connect the ko.

Instead of responding to a ko threat, a player may also choose to ignore the threat and connect the ko.^[22] They thereby win the ko, but at a cost. The choice of when to respond to a threat and when to ignore it is a subtle one, which requires a player to consider many factors, including how much is gained by connecting, how much is lost by not responding, how many possible ko threats both players have remaining, what the optimal order of playing them is, and what the size—points lost or gained—of each of the remaining threats is.

Frequently, the winner of the ko fight does not connect the ko but instead captures one of the chains that constituted their opponent's side of the ko.^[22] In some cases, this leads to another ko fight at a neighboring location.

Strategy[link]

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Game 1 of the 2002 Korean LG Cup final between Choe Myeong-hun (White) and Lee Sedol (Black)^[34] at the end of the opening stage; white has developed a great deal of potential territory, while black has emphasized central influence. After white move 1, black turns at a, and makes thickness all the way out to b, which he then uses to mount a successful attack on the marked white group.

Strategy deals with global influence, interaction between distant stones, keeping the whole board in mind during local fights, and other issues that involve the overall game. It is therefore possible to allow a tactical loss when it confers a strategic advantage.

Go is not easy to play well. With each new level (rank) comes a deeper appreciation for the subtlety and nuances involved and for the insight of stronger players. The acquisition of major concepts of the game comes slowly. Novices often start by randomly placing stones on the board, as if it were a game of chance; they inevitably lose to experienced players who know how to create effective formations. An understanding of how stones connect for greater power develops, and then a few basic common opening sequences may be understood. Learning the ways of life and death helps in a fundamental way to develop one's strategic understanding of weak groups.^{[nb 6]} It is necessary to play thousands of games before one can get close to one's ultimate potential skill level in Go. A player who both plays aggressively and can handle adversity is said to display kiai, or fighting spirit, in the game.

Familiarity with the board shows first the tactical importance of the edges, and then the efficiency of developing in the corners first, then sides, then center. The more advanced beginner understands that territory and influence are somewhat interchangeable—but there needs to be a balance. This intricate struggle of power and control makes the game highly dynamic.

Basic concepts[link]

Basic strategic aspects include the following:

• Connection: Keeping one's own stones connected means that fewer groups need to make living shape, and one has fewer groups to defend.
• Cut: Keeping opposing stones disconnected means that the opponent needs to defend and make living shape for more groups.
• Stay alive: The simplest way to stay alive is to establish a foothold in the corner or along one of the sides. At a minimum, a group must have two eyes (separate open points) to be "alive". An opponent cannot fill in either eye, as any such move is suicidal and prohibited in the rules.
• Mutual life (seki) is better than dying: A situation in which neither player can play to a particular point without then allowing the other player to play at another point to capture. The most common example is that of adjacent groups that share their last few liberties—if either player plays in the shared liberties, they can reduce their own group to a single liberty (putting themselves in atari), allowing their opponent to capture it on the next move.
• Death: A group that lacks living shape, meaning less than two eyes is eventually removed from the board as captured.
• Invasion: Set up a new living group inside an area where the opponent has greater influence, means one reduces the opponents score in proportion to the area one occupies.
• Reduction: Placing a stone far enough into the opponent's area of influence to reduce the amount of territory they eventually get, but not so far in that it can be cut off from friendly stones outside.
• Sente: A play that forces one's opponent to respond (gote), such as placing an opponent's group in atari (immediate danger of capture). A player who can regularly play sente has the initiative, as in chess, and can control the flow of the game.
• Sacrifice: Allowing a group to die in order to carry out a play, or plan, in a more important area.

The strategy involved can become very abstract and complex. High-level players spend years improving their understanding of strategy, and a novice may play many hundreds of games against opponents before being able to win regularly.

Opening strategy[link]

In the opening of the game, players usually play in the corners of the board first, as the presence of two edges make it easier for them to surround territory and establish their stones.^[35] After the corners, focus moves to the sides, where there is still one edge to support a player's stones. Opening moves are generally on the third and fourth line from the edge, with occasional moves on the second and fifth lines. In general, stones on the third line offer stability and are good defensive moves, whereas stones on the fourth line influence more of the board and are good attacking moves.

In the opening, players often play established sequences called joseki, which are locally balanced exchanges;^[36] however, the joseki chosen should also produce a satisfactory result on a global scale. It is generally advisable to keep a balance between territory and influence. Which of these gets precedence is often a matter of individual taste.

Phases of the game[link]

While the opening moves in a game have a distinct set of aims, they usually make up only 10% to at most 20% of the game. In other words, in a game of 250 moves, there may be around 30 or so opening moves, with limited "fighting". At the end of such a game, around the last 100 moves are considered the endgame, in which territories are finished off definitively and all issues on capturing stones become clear. The middle phase of the game is the most combative, and usually lasts for more than 100 moves. During the middlegame, or just "the fighting", the players invade each others' frameworks, and attack weak groups, formations that lack the necessary two eyes for viability. Such groups must run away, i.e., expand to avoid enclosure, giving a dynamic feeling to the struggle. It is possible that one player may succeed in capturing a large weak group of the opponent's, which often proves decisive and ends the game by a resignation. However, matters may be more complex yet, with major trade-offs, apparently dead groups reviving, and skillful play to attack in such a way as to construct territories rather than kill.

The end of the middlegame and transition to the endgame is marked by a few features. The game breaks up into areas that do not affect each other (with a caveat about ko fights), where before the central area of the board related to all parts of it. No large weak groups are still in serious danger. Moves can reasonably be attributed some definite value, such as 20 points or fewer, rather than simply being necessary to compete. Both players set limited objectives in their plans, in making or destroying territory, capturing or saving stones. These changing aspects of the game usually occur at much the same time, for strong players. In brief, the middlegame switches into the endgame when the concepts of strategy and influence need reassessment in terms of concrete final results on the board.

History[link]

Origin in China[link]

Woman Playing Go (Tang Dynasty ca. 744), discovered at the Astana Graves

The earliest written reference to the game is generally recognized as the historical annal Zuo Zhuan^[37] (c. 4th century BC),^[38] referring to a historical event of 548 BC. It is also mentioned in Book XVII of the Analects of Confucius^[38] and in two books written by Mencius^[39] (c. 3rd century BC).^[38] In all of these works, the game is referred to as yì (弈). Today, in China, it is known as weiqi (simplified Chinese: 围棋; traditional Chinese: 圍棋; pinyin: wéiqí; Wade–Giles: wei ch'i), literally the "encirclement board game".

Go was originally played on a 17×17 line grid, but a 19×19 grid became standard by the time of the Tang Dynasty (618–907).^[40] Legends trace the origin of the game to Chinese emperor Yao (2337–2258 BC), said to have had his counselor Shun design it for his unruly son, Danzhu, to favorably influence him.^[41] Other theories suggest that the game was derived from Chinese tribal warlords and generals, who used pieces of stone to map out attacking positions.^[42]

In China, Go was considered one of the four cultivated arts of the Chinese scholar gentleman, along with calligraphy, painting and playing the musical instrument guqin.^[43]

Spread to Korea and Japan[link]

Weiqi was introduced to Korea sometime between the 5th and 7th centuries AD, and was popular among the higher classes. In Korea, the game is called baduk (hangul: 바둑), and a variant of the game called Sunjang baduk was developed by the 16th century. Sunjang baduk became the main variant played in Korea until the end of the 19th century.^[44][45]

Korean players, in traditional dress, play in a photograph dated between 1910 and 1920.

The game reached Japan in the 7th century AD—where it is called go (碁^?) or igo (囲碁^?)—the game became popular at the Japanese imperial court in the 8th century,^[46] and among the general public by the 13th century.^[47] In 1603, Tokugawa Ieyasu re-established Japan's unified national government. In the same year, he assigned the then-best player in Japan, a Buddhist monk named Nikkai (né Kanō Yosaburo, 1559), to the post of Godokoro (Minister of Go).^[48] Nikkai took the name Honinbo Sansa and founded the Honinbo Go school.^[48] Several competing schools were founded soon after.^[48] These officially recognized and subsidized Go schools greatly developed the level of play and introduced the dan/kyu style system of ranking players.^[49] Players from the four schools (Honinbo, Yasui, Inoue and Hayashi) competed in the annual castle games, played in the presence of the shogun.^[50]

Go in the West[link]

Despite its widespread popularity in East Asia, Go has been slow to spread to the rest of the world. Although there are some mentions of the game in western literature from the 16th century forward, Go did not start to become popular in the West until the end of the 19th century, when German scientist Oskar Korschelt wrote a treatise on the game.^[51] By the early 20th century, Go had spread throughout the German and Austro-Hungarian empires. In 1905, Edward Lasker learned the game while in Berlin. When he moved to New York, Lasker founded the New York Go Club together with (amongst others) Arthur Smith, who had learned of the game while touring the East and had published the book The Game of Go in 1908.^[52] Lasker's book Go and Go-moku (1934) helped spread the game throughout the US,^[52] and in 1935, the American Go Association was formed. Two years later, in 1937, the German Go Association was founded.

World War II put a stop to most Go activity, but after the war, Go continued to spread.^[53] For most of the 20th century, the Japan Go Association played a leading role in spreading Go outside East Asia by publishing the English-language magazine Go Review in the 1960s; establishing Go centers in the US, Europe and South America; and often sending professional teachers on tour to Western nations.^[54]

In 1996, NASA astronaut Daniel Barry and Japanese astronaut Koichi Wakata became the first people to play Go in space. They used a special Go set, which was named Go Space, designed by Wai-Cheung Willson Chow.^[55] Both astronauts were awarded honorary dan ranks by the Nihon Ki-in.^[55]

As of 2008^[update], the International Go Federation has a total of 71 member countries.^[56] It has been claimed that across the world 1 person in every 222 plays Go.^[57]

Competitive play[link]

Ranks and ratings[link]

In Go, rank indicates a player's skill in the game. Traditionally, ranks are measured using kyu and dan grades,^[58] a system also adopted by many martial arts. More recently, mathematical rating systems similar to the Elo rating system have been introduced.^[59] Such rating systems often provide a mechanism for converting a rating to a kyu or dan grade.^[59] Kyu grades (abbreviated k) are considered student grades and decrease as playing level increases, meaning 1st kyu is the strongest available kyu grade. Dan grades (abbreviated d) are considered master grades, and increase from 1st dan to 7th dan. First dan equals a black belt in eastern martial arts using this system. The difference among each amateur rank is one handicap stone. For example, if a 5k plays a game with a 1k, the 5k would need a handicap of four stones to even the odds. Top-level amateur players sometimes defeat professionals in tournament play.^[60] Professional players have professional dan ranks (abbreviated p). These ranks are separate from amateur ranks.

The rank system comprises, from the lowest to highest ranks:

Tournament and match rules[link]

Tournament and match rules deal with factors that may influence the game but are not part of the actual rules of play. Such rules may differ between events. Rules that influence the game include: the setting of compensation points (komi), handicap strategies, and time control parameters. Rules that do not generally influence the game are: the tournament system, pairing strategies, and placement criteria.

Common tournament systems used in Go include the McMahon system,^[61] Swiss system, league systems and the knockout system. Tournaments may combine multiple systems; many professional Go tournaments use a combination of the league and knockout systems.^[62]

Tournament rules may also set the following:

• compensation points, called komi, which compensate the second player for the first move advantage of his opponent; tournaments commonly use a compensation in the range of 5–8 points,^[63] generally including a half-point to prevent draws;
• compensation stones placed on the board before alternate play, allowing players of different strengths to play competitively (see Go handicap for more information); and
• superko: Although the basic ko rule described above covers more than 95% of all cycles occurring in games,^[64] there are some complex situations—triple ko, eternal life,^{[nb 7]} etc.—that are not covered by it but would allow the game to cycle indefinitely. To prevent this, the ko rule is sometimes extended to disallow the repetition of any previous position. This extension is called superko.^[64]

Top players[link]

Although the game was developed in China, the establishment of the Four Go houses by Tokugawa Ieyasu at the start of the 17th century shifted the focus of the Go world to Japan. State sponsorship, allowing players to dedicate themselves full time to study of the game, and fierce competition between individual houses resulted in a significant increase in the level of play. During this period, the best player of his generation was given the prestigious title Meijin (master) and the post of Godokoro (minister of Go). Of special note are the players who were dubbed Kisei (Go Sage). The only three players to receive this honor were Dosaku, Jowa and Shusaku, all of the house Honinbo.^[65]

Honinbo Shusai (left), last head of house Honinbo, plays against then-up-and-coming Go Seigen in the game of the century.

After the end of the Tokugawa shogunate and the Meiji Restoration period, the Go houses slowly disappeared, and in 1924, the Nihon Ki-in (Japanese Go Association) was formed. Top players from this period often played newspaper-sponsored matches of 2–10 games.^[66] Of special note are Go Seigen (Chinese: Wu Qingyuan), who scored an impressive 80% in these matches,^[67] and Minoru Kitani, who dominated matches in the early 1930s.^[68] These two players are also recognized for their groundbreaking work on new opening theory (Shinfuseki).^[69]

For much of the 20th century, Go continued to be dominated by players trained in Japan. Notable names included Eio Sakata, Rin Kaiho (born in China), Masao Kato, Koichi Kobayashi and Cho Chikun (born Cho Ch'i-hun, South Korea).^[70] Top Chinese and Korean talents often moved to Japan, because the level of play there was high and funding was more lavish. One of the first Korean players to do so was Cho Namchul, who studied in the Kitani Dojo 1937–1944. After his return to Korea, the Hanguk Kiwon (Korea Baduk Association) was formed and caused the level of play in South Korea to rise significantly in the second half of the 20th century.^[71] In China, the game declined during the Cultural Revolution (1966–1976) but quickly recovered in the last quarter of the 20th century, bringing Chinese players, such as Nie Weiping and Ma Xiaochun, on par with their Japanese and Korean counterparts.^[72]

Korean player Lee Chang-ho, regarded by many as one of the best modern Go players, plays against Russian player Alexandre Dinerchtein, seven-time European Champion and one of the few Western players to reach professional status.

With the advent of major international titles from 1989 onward, it became possible to compare the level of players from different countries more accurately. Korean players such as Lee Chang-ho, Cho Hunhyun, Lee Sedol and Yoo Changhyuk dominated international Go and won an impressive number of titles.^[73] Several Chinese players also rose to the top in international Go, most notably Ma Xiaochun, Chang Hao and Gu Li. As of 2008^[update], Japan lags behind in the international Go scene.

Historically, as with most sports and games, more men than women have played Go. Special tournaments for women exist, but until recently, men and women did not compete together at the highest levels; however, the creation of new, open tournaments and the rise of strong female players, most notably Rui Naiwei, have in recent years highlighted the strength and competitiveness of emerging female players.^[74]

The level in other countries has traditionally been much lower, except for some players who had preparatory professional training in Asia.^{[nb 8]} Knowledge of the game has been scant elsewhere up until the 20th century. A famous player of the 1920s was Edward Lasker.^{[nb 9]}It was not until the 1950s that more than a few Western players took up the game as other than a passing interest. In 1978, Manfred Wimmer became the first Westerner to receive a professional player's certificate from an Asian professional Go association.^[75] In 2000, a Westerner, Michael Redmond, finally achieved the top rank awarded by an Asian Go association, 9 dan. In total, as of 2008^[update], only nine non-Asian Go players have ever achieved professional status in Asian associations.

Equipment[link]

A traditional Japanese set, with floor board (碁盤 goban), bowls (碁笥 goke) and stones (碁石 goishi)

It is possible to play Go with a simple paper board and coins or plastic tokens for the stones. More popular midrange equipment includes cardstock, a laminated particle board, or wood boards with stones of plastic or glass. More expensive traditional materials are still used by many players.The most expensive Go sets have black stones carved from slate and white stones carved from translucent white shells, played on boards carved in a single piece from the trunk of a tree.

Traditional equipment[link]

Boards[link]

The Go board (generally referred to by its Japanese name goban) typically measures between 45 and 48 cm (18 and 19 in) in length (from one player's side to the other) and 42 to 44 cm (17 to 17 in) in width. Chinese boards are slightly larger, as a traditional Chinese Go stone is slightly larger to match. The board is not square; there is a 15:14 ratio in length to width, because with a perfectly square board, from the player's viewing angle the perspective creates a foreshortening of the board. The added length compensates for this.^[76] There are two main types of boards: a table board similar in most respects to other gameboards like that used for chess, and a floor board, which is its own free-standing table and at which the players sit.

The traditional Japanese goban is between 10 and 18 cm (3.9 and 7.1 in) thick and has legs; it sits on the floor (see picture to right).^[76] It is preferably made from the rare golden-tinged Kaya tree (Torreya nucifera), with the very best made from Kaya trees up to 700 years old. More recently, the related California Torreya (Torreya californica) has been prized for its light color and pale rings as well as its reduced expense and more readily available stock. The natural resources of Japan have been unable to keep up with the enormous demand for the slow-growing Kaya trees; both T. nucifera and T. californica take many hundreds of years to grow to the necessary size, and they are now extremely rare, raising the price of such equipment tremendously.^[77] As Kaya trees are a protected species in Japan, they cannot be harvested until they have died. Thus, an old-growth, floor-standing Kaya goban can easily cost in excess of US$10,000 with the highest-quality examples costing more than $60,000.^[78]

Other, less expensive woods often used to make quality table boards in both Chinese and Japanese dimensions include Hiba (Thujopsis dolabrata), Katsura (Cercidiphyllum japonicum), Kauri (Agathis), and Shin Kaya (various varieties of spruce, commonly from Alaska, Siberia and China's Yunnan Province).^[77] So-called Shin Kaya is a potentially confusing merchant's term: shin means "new", and thus shin kaya is best translated "faux kaya", because the woods so described are biologically unrelated to Kaya.^[77]

Stones[link]

A full set of Go stones (goishi) usually contains 181 black stones and 180 white ones; a 19×19 grid has 361 points, so there are enough stones to cover the board, and Black gets the extra odd stone because that player goes first.

Traditional Japanese stones are double-convex, and made of clamshell (white) and slate (black).^[79] The classic slate is nachiguro stone mined in Wakayama Prefecture and the clamshell from the Hamaguri clam; however, due to a scarcity in the Japanese supply of this clam, the stones are most often made of shells harvested from Mexico.^[79] Historically, the most prized stones were made of jade, often given to the reigning emperor as a gift.^[79]

In China, the game is traditionally played with single-convex stones^[79] made of a composite called Yunzi. The material comes from Yunnan Province and is made by sintering a proprietary and trade-secret mixture of mineral compounds derived from the local stone. This process dates to the Tang Dynasty and, after the knowledge was lost in the 1920s during the Chinese Civil War, was rediscovered in the 1960s by the now state-run Yunzi company. The material is praised for its colors, its pleasing sound as compared to glass or to synthetics such as melamine, and its lower cost as opposed to other materials such as slate/shell. The term "yunzi" can also refer to a single-convex stone made of any material; however, most English-language Go suppliers specify Yunzi as a material and single-convex as a shape to avoid confusion, as stones made of Yunzi are also available in double-convex while synthetic stones can be either shape.

Traditional stones are made so that black stones are slightly larger in diameter than white; this is to compensate for the optical illusion created by contrasting colors that would make equal-sized white stones appear larger on the board than black stones.^{[79][nb 10]}

An example of single-convex stones and Go Seigen bowls. These particular stones are made of Yunzi material, and the bowls of jujube wood.

Bowls[link]

The bowls for the stones are shaped like a flattened sphere with a level underside.^[80] The lid is loose fitting and upturned before play to receive stones captured during the game. Chinese bowls are slightly larger, and a little more rounded, a style known generally as Go Seigen; Japanese Kitani bowls tend to have a shape closer to that of the bowl of a snifter glass, such as for brandy. The bowls are usually made of turned wood. Rosewood is the traditional material for Japanese bowls, but is very expensive; wood from the Chinese jujube date tree, which has a lighter color (it is often stained) and slightly more visible grain pattern, is a common substitute for rosewood, and traditional for Go Seigen-style bowls. Other traditional materials used for making Chinese bowls include lacquered wood, ceramics, stone and woven straw or rattan. The names of the bowl shapes, Go Seigen and Kitani, pay homage to two 20th-century professional Go players by the same names, of Chinese and Japanese nationality, respectively, who are referred to as the "Fathers of modern Go".^[65]

Modern and low-cost alternatives[link]

In clubs and at tournaments, where large numbers of sets must be purchased and maintained by one organization, expensive traditional sets are not usually used. For these situations, table boards are usually used instead of floor boards, and are either made of a lower-cost wood such as spruce or bamboo, or are flexible mats made of vinyl or leather that can be rolled up. In such cases, the stones are usually made of glass, plastic or resin (such as melamine or Bakelite) rather than slate and shell. Bowls are often made of plastic or inexpensive wood.

Common "novice" Go sets are all-inclusive kits made of particle board or plywood, with plastic or glass stones, that either fold up to enclose the stone containers or have pull-out drawers to keep stones. In relative terms, these sets are inexpensive, costingUS$20–$40 depending on component quality, and thus are popular with casual Go players. Magnetic sets are also available, either as portable travel sets or in larger sizes for educational purposes.

Playing technique and etiquette[link]

Go players in Shanghai demonstrate the traditional technique of holding a stone.

The traditional way to place a Go stone is to first take one from the bowl, gripping it between the index and middle fingers, with the middle finger on top, and then placing it directly on the desired intersection.^[81] It is considered respectful towards one's opponent to place one's first stone in the upper right-hand corner.^[82]

It is considered poor manners to run one's fingers through one's bowl of unplayed stones, as the sound, however soothing to the player doing this, can be disturbing to one's opponent. Similarly, "clacking" a stone against another stone, the board, or the table or floor is also discouraged. However, it is permissible to emphasize select moves by striking the board more firmly than normal, thus producing a sharp clack.

Time control[link]

A game of Go may be timed using a game clock. Formal time controls were introduced into the professional game during the 1920s and were controversial.^[83] Adjournments and sealed moves began to be regulated in the 1930s. Go tournaments use a number of different time control systems. All common systems envisage a single main period of time for each player for the game, but they vary on the protocols for continuation (in overtime) after a player has finished that time allowance.^{[nb 11]} The most widely used time control system is the so called byoyomi^{[nb 12]} system. The top professional Go matches have timekeepers so that the players do not have to press their own clocks.

Two widely used variants of the byoyomi system are:^[84]

• Standard byoyomi: After the main time is depleted, a player has a certain number of time periods (typically around thirty seconds). After each move, the number of full time periods that the player took (often zero) is subtracted. For example, if a player has three thirty-second time periods and takes thirty or more (but less than sixty) seconds to make a move, they lose one time period. With 60–89 seconds, they lose two time periods, and so on. If, however, they take less than thirty seconds, the timer simply resets without subtracting any periods. Using up the last period means that the player has lost on time.
• Canadian byoyomi: After using all of their main time, a player must make a certain number of moves within a certain period of time, such as twenty moves within five minutes.^{[84][nb 13]} If the time period expires without the required number of stones having been played, then the player has lost on time.^{[nb 14]}

Notation and recording games[link]

Go games are recorded with a simple coordinate system.This is comparable to algebraic chess notation, except that Go stones do not move and thus require only one coordinate per turn.Coordinate systems include purely numerical (4-4 point), hybrid (K3), and purely alphabetical.^[85]The Smart Game Format uses alphabetical coordinates internally, but most editors represent the board with hybrid coordinates as this reduces confusion.The Japanese word kifu is sometimes used to refer to a game record.

In Unicode, Go stones are encoded in the block Miscellaneous Symbols:

• U+2686 ⚆ white circle with dot right (HTML: ⚆)^[86]
• U+2687 ⚇ white circle with two dots (HTML: ⚇)
• U+2688 ⚈ black circle with dot right (HTML: ⚈)
• U+2689 ⚉ black circle with two dots (HTML: ⚉)

Computers and Go[link]

Nature of the game[link]

In combinatorial game theory terms, Go is a zero sum, perfect-information, partisan, deterministic strategy game, putting it in the same class as chess, checkers (draughts) and Reversi (Othello); however it differs from these in its game play. Although the rules are simple, the practical strategy is extremely complex.

The game emphasizes the importance of balance on multiple levels and has internal tensions. To secure an area of the board, it is good to play moves close together; however, to cover the largest area, one needs to spread out, perhaps leaving weaknesses that can be exploited. Playing too low (close to the edge) secures insufficient territory and influence, yet playing too high (far from the edge) allows the opponent to invade.

It has been claimed that Go is the most complex game in the world due to its vast number of variations in individual games.^[87] Its large board and lack of restrictions allow great scope in strategy and expression of players' individuality. Decisions in one part of the board may be influenced by an apparently unrelated situation in a distant part of the board. Plays made early in the game can shape the nature of conflict a hundred moves later.

The game complexity of Go is such that describing even elementary strategy fills many introductory books. In fact, numerical estimates show that the number of possible games of Go far exceeds the number of atoms in the known universe.^{[nb 15]}

Software players[link]

Go poses a daunting challenge to computer programmers.^[88]The strongest computer chess programs defeat the best human players (for example, the Deep Fritz program, running on a laptop, beat reigning world champion Vladimir Kramnik without losing a single game in 2006). The best Go programs manage to reach amateur dan level.^[89] On the small 9×9 board, the computer fares better, and some programs now win a fraction of their 9×9 games against professional players. Many in the field of artificial intelligence consider Go to require more elements that mimic human thought than chess.^[90]

A finished beginner's game on a 13×13 board. Go software can reach stronger levels on a smaller board size.

The reasons computer programs do not play Go at the professional dan level:^[91]

• The number of spaces on the board is much larger (over five times the spaces on a chess board—361 vs. 64). On most turns there are many more possible moves in Go than in chess. Throughout most of the game, the number of legal moves stays at around 150–250 per turn, and rarely goes below 50 (in chess, the average number of moves is 37).^[92] Because an exhaustive computer program for Go must calculate and compare every possible legal move in each ply (player turn), its ability to calculate the best plays is sharply reduced when there are a large number of possible moves. Most computer game algorithms, such as those for chess, compute several moves in advance. Given an average of 200 available moves through most of the game, for a computer to calculate its next move by exhaustively anticipating the next four moves of each possible play (two of its own and two of its opponent's), it would have to consider more than 320 billion (3.2×10¹¹) possible combinations. To exhaustively calculate the next eight moves, would require computing 512 quintillion (5.12×10²⁰) possible combinations. As of June 2008^[update], the most powerful supercomputer in the world, IBM's "Roadrunner" distributed cluster, can sustain 1.02 petaflops.^[93][94][95] At this rate, even given an exceedingly low estimate of 10 flops required to assess the value of one play of a stone, Roadrunner would require 138 hours, more than five days, to assess all possible combinations of the next eight moves in order to make a single play.
• Unlike chess and Reversi, the placement of a single stone in the initial phase can affect the play of the game hundreds of moves later. For a computer to have a real advantage over a human, it would have to predict this influence, and from the example above, it would be completely unworkable to attempt to exhaustively analyze the next hundred moves to predict what a stone's placement will do.
• In capture-based games (such as chess), a position can often be evaluated relatively easily, such as by calculating who has a material advantage or more active pieces.^{[nb 16]} In Go, there is often no easy way to evaluate a position.^[88][96] The number of stones on the board (material advantage) is only a weak indicator of the strength of a position, and a territorial advantage (more empty points surrounded) for one player might be compensated by the opponent's strong positions and influence all over the board.

As an illustration, the greatest handicap normally given to a weaker opponent is 9 stones. It was not until August 2008 that a computer won a game against a professional level player at this handicap. It was the Mogo program, which scored this first victory in an exhibition game played during the US Go Congress.^[97][98]

Software assistance[link]

A 9×9 game with graphical aids: note the use of colour, grey areas are neutral, Dame and Seki, ghost stone is hint, star for excellent move.

Beyond programs that play Go, there is an abundance of software available to support players of the game. This includes programs that can be used to view or edit game records and diagrams, programs that allow the user to search for patterns in the games of strong players, and programs that allow users to play against each other over the Internet.

Some web servers such as Peepo.com provide graphical aids like maps, to aid learning during play. These graphical aids may suggest possible next moves, indicate areas of influence, highlight vital stones under attack and mark stones in atari or about to be captured.

There are several file formats used to store game records, the most popular of which is SGF, short for Smart Game Format. Programs used for editing game records allow the user to record not only the moves, but also variations, commentary and further information on the game.^{[nb 17]}

Electronic databases can be used to study life and death situations, joseki, fuseki and games by a particular player. Programs are available that give players pattern searching options, which allow players to research positions by searching for high-level games in which similar situations occur. Such software generally lists common follow-up moves that have been played by professionals and gives statistics on win/loss ratio in opening situations.

Internet-based Go servers allow access to competition with players all over the world, for real-time and turn-based games.^{[nb 18]} Such servers also allow easy access to professional teaching, with both teaching games and interactive game review being possible.^{[nb 19]}

Video games[link]

Igo: Kyū Roban Taikyoku (囲碁 九路盤対局^?, lit. "Go Roadbed Opposite Nine")^[99] is a 1987 Go video game that was released exclusively in Japan for the Family Computer.

In culture and science[link]

Literature, television, and film[link]

Apart from technical literature and study material, Go and its strategies have been the subject of several works of fiction, such as The Master of Go by Nobel prize-winning author Yasunari Kawabata^{[nb 20]} and The Girl Who Played Go by Shan Sa. Other books have used Go as a theme or minor plot device. For example, the 1979 novel Shibumi by Trevanian centers around the game and uses Go metaphors.^[100] Go, referred to as Weiqi, features prominently in the 1986 Eric Van Lustbader novel Jian.

Similarly, Go has been used as a subject or plot device in film, such as π, A Beautiful Mind and Tron: Legacy. Also in The Go Master, a biopic of Go professional Go Seigen.^{[101][nb 21]} In King Hu's wuxia film The Valiant Ones, the characters are color-coded as Go pieces (black or other dark shades for the Chinese, white for the Japanese invaders), Go boards and stones are used by the characters to keep track of soldiers prior to battle, and the battles themselves are structured like a game of Go.^[102] Go is used as a device for criminal profiling in the pilot episode of Criminal Minds, "Extreme Aggressor".

In the television series Andromeda, flashbacks revealed that Captain Hunt used to play Go with his first officer, Gaheris Rhade.Hunt once caught Rhade cheating at the game by removing some of Hunt's stones when Hunt wasn't looking.Rhade's explanation for his cheating—that losing at a competition would call into question the quality of his genes, which would be a horrible outcome for a Nietzschean—was partially intended as a warning to Hunt that Rhade would later betray him.