Rules of Go

This is an in-depth discussion of the rules of Go. It is possible to state a set of rules for go very succinctly, as is done in the initial outline of the game given below. However, some rule sets contain complex clarifications of the status of rarely occurring positions and other issues. These differences only rarely have strategic consequences, and certainly do not change the overall nature of the game. Beginners need not concern themselves with such minutiae when first learning to play —in fact many strong players do not fully understand them!— but they may be of interest to more experienced players.

Basic rules

Players

Rule: Go is a game between two players, called Black and White.

The choice of black or white is traditionally done by chance between players of even strength. The method of selection is called nigiri.

When players are of different strengths, the weaker player takes black. Black may also have handicap stones (see below).

The equipment

The board

The board, or "goban"

Rule: Go is played on a board. The board is a grid of 19 horizontal and 19 vertical lines.

Definition: A point on the grid where a horizontal line meets a vertical line is called an intersection. Two intersections are said to be adjacent if they are connected by a horizontal or vertical line and have no other intersections between them.

There are 361 intersections on a regular 19 × 19 board.

Each of the following diagrams shows two intersections on a 5 × 5 board:

Adjacent intersections.		Adjacent intersections.		Non-adjacent intersections.

In the first two diagrams, the intersections are adjacent; in the third they are not.

Though 19 × 19 boards are standard, go can be played on another size board. Particularly common sizes for quick games are 9 × 9 and 13 × 13. (See also "Board size" below.)

Beginners might prefer to play on a 9 × 9 board to start. The nature of the game remains similar enough to make this worthwhile, yet the games are shorter. For beginners, playing longer games is less important than playing a greater number of games.

Stones

Rule: Go is played with black stones and white stones. Black has at his disposal an unlimited supply of black stones, and White an unlimited supply of white stones.

The state of the board

Positions

Rule: At any time in the game, each intersection on the board is in one and only one of the following three states: 1) empty; 2) occupied by a black stone; or 3) occupied by a white stone.

The diagram shows a possible position:

Chains

Definition: Two stones of the same color are said to be connected, or to belong to the same chain, if it is possible to pass from one to the other through a succession of adjacent stones of the same color.

Let us examine once again the position above.

In the diagram, stones marked with the same number belong to the same chain. Black and White each have four chains.

Liberties

Definition: A liberty of a chain is an empty intersection adjacent to one of its stones.

In each diagram, the circled intersections are the liberties of the black chain:

Play

Initial position

Rule: at the beginning of the game, the board is empty.

Turn-based play

Rule: The players alternate. Black moves first.

Before Black plays first, the board is empty. Black makes a first move on one of the intersections. Then White makes a second play somewhere:

Start		Black plays		White plays

The game will continue with Black and White alternating.

Play or pass

Rule: On his turn, a player may either place one stone of his color on an empty intersection ("play") or perform no action ("pass").

A player may always pass. Some plays are, however, illegal. (See "Illegal moves" below.) A play or a pass is called a move.

Some plays will have the effect of removing stones from the board. (See "Capture" below.) Once a stone has been played, it remains on the board, in the same location, until the end of the game or until it has been captured.

The following diagram shows how Black could play:

Usually, passing is beneficial only at the end of the game, when all territory has been claimed and further moves would be useless, or even harmful to a player's position.

Capture

Rule: If a play causes any opposing chains to be without liberties, those chains are removed from the board. The chains removed are said to have been captured.

Before		Black plays		After removal

In most cases a play does not remove a chain. A play might remove a large chain of several stones, or more than one chain.

Illegal moves

No suicide (not present in some variations)

Rule: It is illegal to play in such a way as to cause a chain of one's own color not to have any liberties. The presence or absence of liberties is determined after the removal of any opposing chains that may have been captured by the move.

In fact, if any opposing chain at all has been captured, it is easy to see that this situation can never occur.

This rule is said to prohibit suicide (or self-capture). In rule sets that allow suicide, it is replaced by the following:

Alternative rule: If a player plays in a way that causes a chain of his own color not to have any liberties, even after any captured opposing chains have been removed, then that chain is removed.

The removed chain is then deemed to have been captured by the opponent.

Examples:

In each diagram below, if Black were to play at the circled point, his move would be suicide. In most rule sets, this is illegal. If suicide is legal, then in the second case, Black's move would result in the capture of the marked stones.

Here is an example which is not suicide. Black may play at the circled point (under any rule set), since the capture of the marked white stones results in the black chain at the bottom right acquiring liberties.

Ko

Rule: If recapturing a stone would recreate the board position from the previous move, the position is called ko, and the recapturing move is illegal.

In the position above, Black has just captured a stone at the circled point by playing to the right of that point. White could now recapture Black's stone, and so on in infinite stalemate. The ko rule prevents this situation and enables the game to keep moving. This "basic ko rule" is adequate for the simplest kind of ko, which occurs when two single stones could otherwise recapture each other indefinitely. This situation occurs in most games and often has a major or even decisive effect on the result. To prevent endless repetition, the rule of ko forbids plays that would repeat the previous board position. A player may not recapture on their next move and must play elsewhere (or pass). After a new move, the board position will be different, and the stone may then be recaptured.

Other positions which could lead to an endlessly repeating position are rare enough that many frequent players never encounter them; their treatment depends on what rule set is being used. Under some rule sets, such games may be considered "no result" (as if the game never happened). The AGA and New Zealand rule sets have a "superko" rule forbidding any move (not including passing) that repeats any previous board position. The Ing rules feature a complicated distinction between "fighting" and "disturbing" ko.

The word ko (Japanese: 劫 kō; usually written with katakana コウ) means "kalpa" in Japanese [4], and is pronounced with a double-length "o", making it sound somewhat like 'kou' does in English.

End

Rule: Alternate play ends when a player makes a pass and then his/her opponent also passes.

Position used to score the game

The rules may require that the players "play the game out" entirely. This leads to the simplest set of rules for the game.

In practice, a different rule for ending the game is used to avoid the need to play the game out. Players pass as soon as they believe that there are no adverse consequences to passing. When both players have passed consecutively, they most likely agree on what their respective areas (see below) would be had the game been played out. It is these areas that are used to score the game, not the board position after the two passes. (Some rule sets depart slightly from this principle. See "Counting phase" below.) If the players disagree about their areas, they can play the game out.

Winner

The player with the higher score is the winner. Margin of victory does not matter; winning by one point is as good as winning by 100 points. Different scoring rules exist which in practice almost always determine the same winner, see Scoring section below. You choose a rule set before starting the game.

Scoring

The most prominent difference between rulesets is the scoring method. There are two main scoring systems: territory scoring (the traditional Japanese method) and stone or area scoring (the Chinese method). Care should be taken to distinguish between scoring systems and counting methods. Only two scoring systems are in wide use, but there are two ways of counting using "area" scoring.

Territory scoring

In territory scoring (including Japanese and Korean rules) a player's score is determined by the number of empty locations that player has surrounded minus the number of stones their opponent has captured.

Furthermore, Japanese and Korean rules have special provisions in cases of seki, though this is not a necessary part of a territory scoring system. (See "Seki" below.)

Typically, counting is done by having each player place the prisoners they have taken into the opponent's territory and rearranging the remaining territory into easy-to-count shapes.

Stone or area scoring

In stone or area scoring (including Chinese rules), a player's score is determined by the number of stones that player has on the board plus the empty area surrounded by that player's stones.

There are several common ways in which to count the score (all these ways will always result in the same winner):

• The oldest counting method is as follows: At the end of the game, all white stones are removed from the board, and the players use black stones to fill the entirety of the black territory. Score is determined by counting the black stones. Since the board contains 361 intersections, black must have 181 or more stones to win. This method is still widely used in mainland of China.
• Around 1975, Taiwanese player and industrialist Ing Chang-ki invented a method of counting now known as Ing counting. Each player begins the game with exactly 180 stones (Mr. Ing also invented special stone containers that count each player's stones). At the end, all stones are placed on the board. One vacant intersection will remain, appearing in the winner's area; the number of stones of one color in the other color's area will indicate the margin of victory.

Attempts at reconciling the two scoring systems

If the game ends with both players having passed the same number of times, then the score will be identical no matter which method is used. AGA rules call for a player to give the opponent a stone when passing. This "passing stone" ensures a correct result under any counting method.

Counting phase

Customarily, when players agree that there are no useful moves left (most often by passing in succession), they attempt to agree which groups are alive and which are dead. If disagreement arises, then under Chinese rules the players simply play on.

However, under Japanese rules, the game is already considered to have ended. The players attempt to ascertain which groups of stones would remain if both players played perfectly from that point on. (These groups are said to be alive.) In addition, this play is done under rules in which kos are treated differently from ordinary play. If the players reach an incorrect conclusion, then they both lose.

Unlike most other rule sets, the Japanese rules contain lengthy definitions of when groups are considered alive and when they are dead. In fact, these definitions do not cover every situation that may arise. Some difficult cases not entirely determined by the rules and existing precedent must be adjudicated by a go tribunal.

The need for the Japanese rules to address the definition of life and death follows from the fact that in the Japanese rules, scores are calculated by territory rather than by area. The rules cannot simply require a player to play on in order to prove that an opponent's group is dead, since playing in his own territory to do this would reduce his score. Therefore, the game is divided into a phase of ordinary play, and a phase of determination of life and death (which according to the Japanese rules is not technically part of the game).

Optional rules

Compensation

To allow players of different skills to compete fairly, handicaps and komi are used. These are considered a part of the game and, unlike in many other games, they do not distort the nature of the game. Players at all levels employ handicaps to make the game more balanced.

Komi

Main article: Komidashi

In an "even", or non-handicap game, Black's initial advantage of moving first can be offset by komi (compensation points): a fixed number of points, agreed before the game, added to White's score at the end of the game. The correct value of komi (to properly compensate for Black's advantage) is controversial, but common values are 5.5, 6.5 or 7.5 -- the fractional value avoids a tied game. In a handicap game, komi is usually set to 0.5 (i.e. White wins if the game is tied). A handicap game with a handicap of 1 starts like an even game, but White receives only 0.5 komi (i.e. a White player who is stronger by one rank is handicapped only by Black's first-move advantage).

There was no komi system before the 20th century. When the great Shusaku was once asked how an important game came out, he said simply, "I had Black," implying that victory was inevitable. As more people become aware of the significance of Black having the first move, komi was introduced. The amount of komi has been increased periodically based on analysis of game results indicated >50% wins for Black; each time, after a period of adjustment Black has again begun winning >50% of games. When it was introduced in Japanese Professional games, it was 4.5 points. However, Black still had better chance to win, so komi was increased to 5.5 points in 1974. In 2002, the Japanese Go Association again increased the komi value to 6.5.

Handicap

Main article: Go handicaps

Handicaps are given by allowing the weaker player to take Black, and declaring White's first few moves as mandatory "pass" moves. In practice, this means that Black's first move is to place a set number of stones (usually the number is equal to the difference in the players' ranks) on the board before allowing White to play. Traditionally, the hoshi ("star points") -- strategically-important intersections marked with small dots -- are used to place these handicap stones. On the 19 × 19 board, there are nine star points: at the four 4-4 points in the corners, at the four 4-10 points along the sides, and one at the 10-10 point (the centre of the board, or tengen in Japanese). Smaller boards such as the 13 × 13 and 9 × 9 also have star points. The 13 × 13 has 9 at the 4-4 points, 4-7 points, and the center. The 9 × 9 board has only 5 points: the 3-3 points and the center.

When Black is only one rank weaker (also known as one stone weaker, due to the close relationship between ranks and the handicap system) he or she is given the advantage of playing Black, perhaps without komi, but without any mandatory White passes. For rank differences from two through nine stones, the appropriate number of handicap stones is used. Beyond nine stones, the difference in strength between the players is usually considered great enough that the game is more a lesson, with White teaching Black, so nine stones is the nominal upper limit on handicap stones regardless of the difference in rank (though higher numbers of stones, up to 41 stones in some cases, can be given if the teacher wants more of a challenge).

Thinking times

See the Time control section of the main Go article.

Variations

Go was already an ancient game before its rules were codified, and therefore, although the basic rules and strategy are universal, there are regional variations in some aspects of the rules.

Seki

To define this notion, we must begin with two definitions:

Terminology:

• An eye' is a connected group of one (or more) empty intersections entirely surrounded by a chain or chains of stones of one color.

• A chain of one color is independently alive if it is (or can be made to be) adjacent to two eyes.

These definitions are given only loosely, since a number of complications arise when attempts are made to formalize the notion of life and death.

A group of stones of one color is said to be alive by seki (or in seki) if it is not independently alive, yet cannot be captured by the opponent.

For example, in the diagram above, the black and white groups each have only one eye. Hence they are not independently alive. However, if either Black or White were to play at the circled point, the other side would then capture their group by playing in its eye. In this case both the black and white groups are alive by seki.

In the diagram above, the circled point is not surrounded by stones of a single color, and accordingly is not counted as territory for either side (irrespective of rule set). In more complex cases, as here,

a vacant point may be surrounded by a group of a single color which is in seki. According to Japanese and Korean rules, such a point is nonetheless treated as neutral territory for scoring purposes. Generally, the Japanese and Korean rules only count a vacant point as territory for one color if it is surrounded by a group or groups of that color that are independently alive.

Repetition

The major division in rules to prevent repetition is between the simple ko rule and the super ko rule: the simple ko rule (typically part of the Japanese ruleset) prevents repetition of the last previous board position; while the super ko rule (typically part of Chinese derived rulesets) prevents repetition of any previous position. In both cases, the rule does not however prohibit passing.

The super ko rule is further differentiated into situational super ko (in which the "position" that cannot be recreated includes knowledge of whose turn it is) and positional super ko (which ignores whose turn it is). Natural situational super ko is a variant in which what matters is not whose turn it is, but who created the position (i. e., who made the last move other than a pass.)

The simple ko rule generally requires the inclusion of additional rules to handle other undesirable repetitions (e.g. long cycles which can lead to no result where the game must be replayed).

Suicide

Currently most major rule sets forbid playing such that a play results in that player's own stones being removed from the board, however some rule sets (notably, New Zealand derived rules and Ing rules) allow suicide of more than one stone. This hardly ever occurs in real games, but in certain circumstances a suicidal move may threaten the opponent's eye shape, yielding a ko threat.

Compensation

The major rulesets differ in how handicap stones are placed on the board: free placement (Chinese), where stones can be placed anywhere (as if the player's turn repeated); and fixed placement(Japanese), where tradition dictates the stone placement (according to the handicap). Area scoring rules and territory scoring rules also differ in the compensation given for each handicap stone (since each handicap stone would count under area scoring). Komi (compensation for going first) also varies, ranging from several fixed values (commonly 5.5, 6.5, or 7.5) to various meta-games to determine a value (notably Auction Komi).

Board sizes

Most Go is played on a 19 × 19 board, but 13 × 13 and 9 × 9 are also popular sizes. Historically other board sizes were commonly used (notably 17 × 17, a predecessor of the 19 × 19 board in ancient China). Go is also sometimes played on various novelty sized boards as small as 5 × 5 and larger than 19 × 19. All board sizes have an odd number of lines to ensure that there is a center point, possibly to make mirror go a less attractive strategy. Generally all rules apply to all board sizes, with the exception of handicaps and compensation (whose placement and values vary according to board size).

Issues

In general, there are three closely related issues which have to be addressed by each variation of the rules.

First, how to ensure that the game comes to an end. Players must be able to settle unsettled situations rather than going around in circles. And neither player should be able to drag the game out indefinitely either to avoid losing or to irritate the other player. Possible methods include: the super-ko rule, time control, or placing an upper bound on the number of moves. This is also affected by the scoring method used since territory scoring penalizes extended play after the boundaries of the territories have been settled.

Second, how to decide which player won the game; and should draws (jigo) be allowed. Possible terms to include in the score are: komi, prisoners captured during the game, stones in dead groups on the board at the end of the game, points of territory controlled by a player but not occupied by his stones, his living stones, the number of passes, and the number of disjoint living groups on the board.

Third, how to determine whether a group of stones is alive or dead at the end of the game, and whether protective plays are necessary e.g. connecting a group which would be in atari if all dame were filled. If the players are unable to agree, some rules provide for arbitration using virtual attempts to capture the group. Others allow play to resume until the group is captured or clearly immortal.

Rule sets

There are many official rule sets for playing Go. These vary in significant ways, such as the method used to count the final score, and in very small ways, such as whether the two kinds of "bent four in the corner" positions result in removal of the dead stones automatically at the end of the game or whether the position must be played out, and whether the players must start the game with a fixed number of stones or with an unbounded number.

Rule sets include Japanese, Chinese, AGA (American Go Association), Ing, and New Zealand.

Further detailed information may be found at the following external links.

Japanese rules

These are rules used in Japan and, with some minor differences, in Korea. They are in wide use throughout the West, sometimes known as "territory" rules. The scoring is based on territory and captured stones. At the end of the game, prisoners are placed in the opponent's territory and players rearrange the board so that territories are easy to count, leaving a visual image resembling the game, which some players find aesthetically pleasing. There is no superko (the triple ko leads to an undecided game.) Suicide is always forbidden. Komi is 6.5.

Disagreements about whether certain groups are alive or dead, and about the counting of territory, are resolved in a notoriously complex manner (see "Counting phase," above).

Japanese rules count vacant points in a seki as neutral, even if they are entirely surrounded by stones of a single color.

World Amateur Go Championship Rules

The rules of the World Amateur Go Championship are based on the Japanese rules, with some differences. These rules are sanctioned by the International Go Federation.

Chinese rules

This is the other major set of rules in widespread use, also known as "area" rules. At the end, one player (usually Black) fills in all of his/her captured territory, and the other (White) stones are removed from the board. Prisoners do not count. Black stones are then arranged in groups of ten -- eighteen such groups, plus half the komi, plus at least one additional stone = victory for Black. So for example with a komidashi of 7.5 points, under Chinese rules Black needs at least 185 stones on the board at the end to win. Komidashi is usually 7.5 points.

In the Chinese rules, there is no penalty for playing within one's territory at the end of the game, for example to kill and remove dead enemy groups. Thus passing to signal that one believes that there are no more useful moves may be conceived as simply being a convenient device to accelerate the end of the game - assuming one is not mistaken. The result will always be the same as if the game had been played out entirely.

The fact that disagreements can be resolved by playing on means that Chinese-style rules can be implemented easily without the need for the rules to define what is meant by "living" and "dead" groups.

World Mind Sports Games Rules

The rules of the First World Mind Sports Games, to be held in Beijing in October 2008, are based on the Chinese rules, but are simpler than they are, and also represent a compromise with the Japanese and Korean rules. These rules are sanctioned by the International Go Federation.

These rules use area scoring, and have a komi of 6.5. Black has one further point deducted in the event that White was the first player to pass in the game. This last feature is a compromise with Japanese and Korean rules in that it is similar, in terms of its strategic consequences, to territory scoring. Unlike the Chinese rules, this rule will generally impose a penalty for an additional move at the end of the game within one's territory. In particular, the result of the game may differ by up to a point from what it would have been had both players played it out.

The game normally ends after two consecutive passes, but in the event of disagreement about the score, play resumes in the original order. Once this resumption has occurred, then when two consecutive passes do eventually occur again, play stops and all stones left on the board are deemed alive. Thus after a single disagreement, the players are required to play the game out entirely. (By this point in the game, there is no longer any penalty for making "useless" plays within one's territory to kill dead enemy groups, since the one-point advantage for passing first has already been attributed to one player or the other by the first set of consecutive passes.)

Suicide is forbidden in these rules.

AGA rules

These are used by the American Go Association. Some special rules (like giving the opponent a prisoner when passing) are added, which make the area scoring and territory scoring equal.

The British Go Association adopted the AGA rules, with some minor departures, in April 2008.

Ing rules

The scoring is basically the same as area scoring, but is done with a special technique involving "Ing bowls". Both players must start with exactly 180 stones; the Ing Foundation makes special bowls that allow players to count their stones easily. Prisoners come back to the owner. After the game finishes, both players fill their empty territory with their stones. The one that gets rid of all of them is the winner. White pays Black eight points (komi) by placing four white stones in Black's territory at the beginning of the counting phase. As Black wins ties it is 7.5 in effect. The ko rule makes a distinction between "fighting" and "disturbing" ko. Multi-stone suicide is allowed. This ruleset was invented and promoted by Ing Chang-ki.

Differences

In most cases the differences between the rule sets are negligible. The choice of rule set rarely results in a difference in score of more than one point, and the strategy and tactics of the game are mostly unaffected by the rule set used.

See also

• Go (board game)
• Go concepts
• Go strategy and tactics

External links

Rules comparisons

• A listing of rules pages
• Another listing of rules pages
• Comparison of some go rules summarized in a table
• Go (Weiqi, Baduk) rules
• Go rule dialects

Specific rule sets

• Official American Go Association rules
• Official Japanese rules as amended in 1989
• Official Chinese rules
• Ing's SST laws of Goe
• "Logical" rules (Tromp-Taylor ruleset)
• "Mathematical Rules" of Go by Jean-Claude Chetrit
• "Simple" rules
• A collection of essays assessing and comparing the various rule sets
• Ikeda's rule sets for Go