Editing Go (game) (section)

== Rules ==
{{Main|Rules of Go}}
Aside from the order of play (alternating moves, Black moves first or takes a handicap) and scoring rules, there are essentially only two rules in Go:
* '''Liberty rule''' states that ''every stone remaining on the board must have at least one open point (a ''liberty'') directly orthogonally adjacent (up, down, left, or right)'', '''or''' ''must be part of a connected group that has at least one such open point (liberty) next to it. Stones or groups of stones which lose their last liberty are removed from the board.''
* '''Repetition Rule''' ([[Rules of go#Ko and Superko|the ko rule]]) states that ''a stone on the board must never immediately repeat a previous position of a captured stone, thus only a move elsewhere on the board is permitted that turn.'' Since without this rule such a pattern of the two players repeating their prior moves (capturing stones in same places) could continue indefinitely, this rule prevents a stalemate.

Almost all other information about how the game is played is heuristic, meaning it is learned information about how the patterns of the stones on the board function, rather than a rule. Other rules are specialized, as they come about through different rulesets, but the above two rules cover almost all of any played game.

Although there are some minor differences between rulesets used in different countries,<ref name=RulesComparison>{{Citation | url = https://www.britgo.org/rules/compare.html | title = Comparison of some go rules | author = British Go Association | access-date = 2007-12-20}}</ref> most notably in Chinese and Japanese scoring rules,<ref>{{Citation | url=http://nrich.maths.org/public/viewer.php?obj_id=1470 | publisher = University of Cambridge | author = NRICH Team | title = Going First | access-date = 2007-06-16}}</ref> these differences do not greatly affect the tactics and strategy of the game.

Except where noted, the basic rules presented here are valid independent of the scoring rules used. The scoring rules are explained separately. [[Go terms]] for which there is no ready English equivalent are commonly called by their Japanese names.

=== Basic rules ===
[[File:Go adjacent stones.png|thumb|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.]]
The two players, Black and White, take turns placing stones of their color on the intersections of the board, one stone at a time. The usual board size is a 19×19 grid, but for beginners or for playing quick games,{{sfn|Kim|Jeong|1997|pp=3–4}} the smaller board sizes of 13×13{{sfn|Nihon Kiin|1973|p=22 (Vol. 1)}} and 9×9 are also popular.{{sfn|Moskowitz|2013|p=14}}
The board is empty to begin with.{{sfn|Lasker|1960|p=2}} Black plays first unless given a handicap of two or more stones, in which case White plays first. The players may choose any unoccupied intersection to play on except for those forbidden by the [[#Ko rule|ko]] and [[#Suicide|suicide]] rules (see below). Once played, a stone can never be moved and can be taken off the board only if it is [[#capture|captured]].{{sfn|Nihon Kiin|1973|p=23 (Vol. 1)}} A player may pass their turn, declining to place a stone, though this is usually only done at the end of the game when both players believe nothing more can be accomplished with further play. When both players pass consecutively, the game ends{{sfn|Fairbairn|2004|p=12}} and is then [[#Scoring_rules|scored]].

===Liberties and capture===
[[File:Go capturing.png|thumb|left|The Black stone group has only one liberty (at point A), so it is very vulnerable to capture. If Black plays at A, the chain would then have 3 liberties, and so is much safer. However, if White plays at A first, the Black chain loses its last liberty, and thus it is captured and immediately removed from the board, leaving White's stones as shown to the right.]]
Vertically and horizontally adjacent stones of the same color form a chain (also called a ''string'' or ''group''),{{sfn|Fairbairn|2004|p=7}} forming a discrete unit that cannot then be divided.<ref>{{citation | url = http://nrich.maths.org/public/viewer.php?obj_id=1433 | title = Behind the Rules of Go | last = Matthews | first = Charles | publisher = University of Cambridge | access-date = 2008-06-09}}</ref> 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 they can be connected together by placing a stone on an intersection that is adjacent to two or more chains of the same color.<ref name="Go Board Game pdf">{{cite web|title=Go The Board Game|url=http://kopoint.files.wordpress.com/2012/12/go-the-game.pdf|access-date=20 August 2014|archive-url=https://web.archive.org/web/20130725124759/http://kopoint.files.wordpress.com/2012/12/go-the-game.pdf|archive-date=25 July 2013|url-status=dead}}</ref>

A vacant point adjacent to a stone, along one of the grid lines of the board, is called a ''liberty'' for that stone.{{sfn|Kim|Jeong|1997|p=12}}{{sfn|Fairbairn|2004|p=6}} Stones in a chain share their liberties.{{sfn|Fairbairn|2004|p=7}} 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.{{sfn|Dahl|2001|p=206}}

{{clear}}

=== Ko rule ===
{{main|Ko fight}}
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An example of a situation in which the ko rule applies
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Players are not allowed to make a move that returns the game to the immediately prior position. This rule, called the [[ko rule]], prevents unending repetition (a stalemate).{{sfn|Kim|Jeong|1997|pp=48–49}} As shown in the example pictured: White had a stone where the red circle was, and Black has just captured it by playing a stone at '''1''' (so the White stone has been removed). However, it is readily apparent that now Black's stone at '''1''' is immediately threatened by the three surrounding White stones. If White were allowed to play again on the red circle, it would return the situation to the original one, but the ''ko'' rule forbids that kind of endless repetition. Thus, White is forced to move elsewhere, or pass. If White wants to recapture Black's stone at '''1''', White must attack Black somewhere else on the board so forcefully that Black moves elsewhere to counter that, giving White that chance. If White's forcing move is successful, it is termed "gaining the ''sente''"; if Black responds elsewhere on the board, then White can retake Black's stone at '''1''', and the ''ko'' continues, but this time Black must move elsewhere. A repetition of such exchanges is called a ''ko fight''.{{sfn|Kim|Jeong|1997|pp=144–147}} To stop the potential for ''ko fights'', two stones of the same color would need to be added to the group, making either a group of 5 Black or 5 White stones.

While the various rulesets 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 {{section link|Rules of Go|Repetition}} for further information.

=== Suicide ===
[[File:Gochsurule.png|thumb|Under normal rules, White cannot play at A because that point has no liberties. Under the Ing<ref name="ing_rules">Ing rules of Go, translation archived by [[American Go Association]] website [https://www.usgo-archive.org/files/pdf/IngRules2006.pdf], {{webarchive|url=https://web.archive.org/web/20130112214116/http://www.usgo.org/files/pdf/IngRules2006.pdf|date=12 January 2013}}, retrieved 5 August 2012</ref> and New Zealand rules,<ref name="AGA_rules">{{Cite web|title=The Rules of Go |website=American Go Association|url=https://www.usgo-archive.org/rules-of-go|access-date=5 August 2012|archive-url=https://web.archive.org/web/20120711140721/http://www.usgo.org/rules-go|archive-date=11 July 2012}}</ref> White may play A, a suicide stone that kills itself and the two neighboring white stones, leaving an empty three-space eye. Black naturally answers by playing at A, creating two eyes to live.]]

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 second case, the enemy group is captured, leaving the new stone with at least one liberty, so the new stone can be placed.{{sfn|Kim|Jeong|1997|p=30}} 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. (An [[List of Go terms|eye]] is an empty point or group of points surrounded by a group of stones).

The [[Rules of go#Ing rules|Ing]] and New Zealand rules do not have this rule,<ref name="Suicide in different rules.">{{cite web|title=Comparison of Some Go Rules|url=https://www.britgo.org/rules/compare.html|publisher=British Go Association|access-date=15 May 2014}}</ref> and there a player might destroy one of its own groups (commit suicide). This play would only be useful in limited sets of situations involving a small interior space or planning.{{sfn|Kim|Jeong|1997|p=28}} In the example at right, it may be useful as a [[#Ko fighting|ko threat]].

=== Komi ===
{{Main|Komi (Go)}}
Because Black has the advantage of playing the first move, the idea of awarding White some compensation came into being during the 20th century. This is called [[komidashi|komi]], which gives white a 5.5-point compensation under Japanese rules, 6.5-point under Korean rules, and 15/4 stones, or 7.5-point under Chinese rules (number of points varies by rule set).<ref name="Komi Change">{{cite web|title=A change in Komi|url=http://www.usgo.org/aga-memo-regarding-komi|access-date=31 May 2014|url-status=dead|archive-url=https://web.archive.org/web/20221229064855/http://www.usgo.org/aga-memo-regarding-komi|archive-date=29 December 2022}}</ref> Under handicap play, White receives only a 0.5-point komi, to break a possible tie (''jigo'').

=== Scoring rules ===
[[File:Gofin.png|thumb|A simplified game at its end. Black's territory (A) + (C) and prisoners (D) is counted and compared to White's territory (B) only (no prisoners). 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 procedures are used, and players determine which to use before play. Both procedures almost always give the same winner.

* '''Area scoring procedure (including Chinese):''' counts the number of points a player's 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.<ref>{{cite web|work=New in Go|title=The rules debate as seen from Ancient China|last=Fairbairn|first=John|date=June 2006|publisher=Games of Go on Disc (GoGoD)|url=http://www.gogod.co.uk/NewInGo/C&IP.htm|access-date=2007-11-27|url-status=dead|archive-url=https://web.archive.org/web/20130112213532/http://www.gogod.co.uk/NewInGo/C%26IP.htm|archive-date=2013-01-12}}</ref> Beginner-friendly, but takes longer to count. A player's score is the number of stones that the player has on the board, plus the number of empty intersections surrounded by that player's stones. 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.
* '''Territory scoring procedure (including Japanese and Korean):''' counts the number of empty points a player's stones surround, together with the number of stones the player captured. 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.{{efn|1=Exceptionally, in Japanese and Korean rules, empty points, even those surrounded by stones of a single color, may count as neutral territory if some of them are alive by seki. See the section below on [[#Seki (mutual life)|seki]].}} Under territory scoring there can be an extra penalty for playing inside ones' territory, so if there is a disagreement extra play to resolve it would, in tournament settings, happen on a separate board, where the player claiming a group is dead would play first, and would demonstrate how to capture those stones. For further information, see [[Rules of Go]].

Both procedures are counted after both players have passed consecutively, the stones that are still on the board but unable to avoid capture, called ''dead'' stones, are removed. Given that the number of stones a player has on the board is directly related to the number of prisoners their opponent has taken, the resulting net score, that is, the difference between Black's and White's scores is identical under both rulesets (unless the players have passed different numbers of times during the course of the game). Thus, the net result given by the two scoring systems rarely differs by more than a point.<ref>{{Citation | url = https://www.cs.cmu.edu/~wjh/go/rules/AGA.commentary.html | title = Demonstration of the Relationship of Area and Territory Scoring | first = Fred | last = Hansen | publisher = American Go Association | access-date = 2008-06-16}}</ref>

=== Life and death ===
{{See also|Life and death}}
While not actually mentioned in the rules of Go (at least in simpler rule sets, such as those of New Zealand and the U.S.), the concept of a ''living'' group of stones is necessary for a practical understanding of the game.{{sfn|Matthews|2002|p={{page needed|date=May 2014}}}}

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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. The point marked ''a'' is a false eye, thus the black group with false eye ''a'' can be killed by white in two turns.
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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: the defending player can make it alive or the opponent can ''kill'' it, depending on who gets to play first.{{sfn|Matthews|2002|p={{page needed|date=May 2014}}}}

An [[List of Go terms|eye]] is an empty point or group of points surrounded by a group of stones. If the eye is surrounded by Black stones, White cannot play there unless such a play would take Black's last liberty and capture the Black stones. (Such a move is forbidden according to the suicide rule in most rule sets, but even if not forbidden, such a move would be a useless suicide of a White stone.)

If a Black group has two eyes, White can never capture it because White cannot remove both liberties simultaneously. If Black has only one eye, White can capture the Black group by playing in the single eye, removing Black's last liberty. Such a move is not suicide because the Black stones are removed first. 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 marked ''a'' is not actually an eye. White can play there and take a black stone. Such a point is often called a ''false eye''.{{sfn|Matthews|2002|p={{page needed|date=May 2014}}}}

=== Seki (mutual life) ===
[[File:Goseki.png|thumb|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).]]

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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).
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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 colored 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 players' stones remain on the board (in seki). Neither player receives any points for those groups, but at least those groups themselves remain living, as opposed to being captured.{{efn|1=In game theoretical terms, seki positions are an example of a [[Nash equilibrium]].}}

Seki can occur in many ways. The simplest are:
# each player has a group without eyes and they share two liberties, and
# each player has a group with one eye and they share one more liberty.
In the "Example of seki (mutual life)" diagram, the two circled points are liberties shared by both a black and a white group. Both of these interior groups are at risk, and neither player wants to play on a circled point, because doing so would allow the opponent to capture their group on the next move. The outer groups in this example, both black and white, are alive. Seki can result from an attempt by one player to invade and kill a nearly settled group of the other player.{{sfn|Matthews|2002|p={{page needed|date=May 2014}}}}