Editing Rules of Go (section)

== 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 ===
For defining this notion, it is useful to introduce some additional terminology:

;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.
{{Go board 5x5
| w| w| b| b| b
| w| w| b| b| b
| w|  | w| b| r
| w| w| w| b| b
| w| w|dc| b| b|24}}

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 ruleset). In more complex cases, as here,<ref>Diagram taken from Robert Jasiek [http://home.snafu.de/jasiek/diffasts.html [s n a f u &#93;] {{in lang|de}}</ref>
{{Go board 5x5
| w| u| b|uc| b
| l| w| b| b| b
| b| b| w| w| w
|lc| b| w|  | w
| b| b| w| w|dr|24}}

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 superko rule (typically part of Chinese derived rulesets, including those of the [[American Go Association|AGA]] and the [[New Zealand Go Society]]) prevents repetition of ''any'' previous position. In both cases, the rule does not, however, prohibit passing.

The super ko rule is differentiated into situational super ko (SSK, in which the "position" that cannot be recreated includes knowledge of whose turn it is) and positional super ko (PSK, which ignores whose turn it is). Natural situational super ko (NSSK) 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 Ing rules feature a complicated distinction between "fighting" and "disturbing" ko.

Situations other than ko which could lead to an endlessly repeating position are rare enough that many frequent players never encounter them; their treatment depends on what ruleset is being used. 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).

The first position below is an example of a ''triple ko'', taken, with minor changes, from Ikeda Toshio's ''On the Rules of Go''.<ref>[http://gobase.org/studying/rules/ikeda/?sec=e2030601 European Go Culture Centre Homepage]</ref>

{|border="0" cellspacing="0" cellpadding="0"
|-valign="top"
|
{{Goban 9x9

| | | | | | | | | 
|b|b|b|b|b|b|b| | 
|w|w|w|w|w|w|b| | 
| |w| |w|b|w|b|b| 
|w|b|w|b| |b|w|b| 
|b|b|b|b|b|b|w|b|b
| |w|w|w|w|w|w|w|w
|w|w| |w| | | |w| 
| | | |w| | | |w|

|24|u=u|d=d|l=l|r=r}}
|&nbsp;&nbsp;
|
{{Goban 9x9

| | | | | | | | | 
|b|b|b|b|b|b|b| | 
|w|w|w|w|w|w|b| | 
| |w| |w|b|w|b|b| 
|w|b|w|b| |b|w|b| 
|b|b|b|b|b|b|w|b|b
| |w|w|w|w|w|w|w|w
|w|w|b|w|b|b|b|w| 
|b|b| |w|b| |b|w|b

|24|u=u|d=d|l=l|r=r}}
|&nbsp;&nbsp;
|
{{Goban 9x9

| | | | | | | | | 
|b|b|b|b|b|b|b| | 
|w|w|w|w|w|w|b| | 
| |w| |w|b|w|b|b| 
|w|b|w|b| |b|w|b| 
|b|b|b|b|b|b|w|b|b
| |w|w|w|w|w|w|w|w
|w|w| |w| |w| |w| 
| |w|w|w|w|w|w|w|w

|24|u=u|d=d|l=l|r=r}}
|-valign="top"
|Triple ko; Black kills group (PSK, SSK)
|&nbsp;&nbsp;
|Result if White moves first (NSSK) 
|&nbsp;&nbsp;
|White wins here with first move. (NSSK)
|}

Without a superko rule, this position would lead to an endless cycle, and hence "no result", a draw, or some other outcome determined by the rules.

For simplicity, when discussing this position using the superko rule, it is assumed that the last move placed a stone in a position unoccupied since the beginning of the game, and away from the ko. Under positional and situational super ko, Black captures the white group. This is also the case with natural situational super ko if it is Black's turn. If it is White's turn however, then NSSK exhibits odd behavior. White can get a seki by passing, but only at the cost of allowing Black unlimited moves away from the ko. If White insists on saving their group, the final position might look like the second diagram. On the other hand, with the first move (which should be a pass), White wins by two points in the third position using NSSK (assuming area scoring). Black's best response, in terms of maximizing their score, is a pass.

=== Suicide ===
Currently, most major rulesets forbid playing such that a play results in that player's own stones being removed from the board. Some rulesets (notably, New Zealand derived rules and Ing rules) allow suicide of more than one stone. Suicide of more than one stone rarely occurs in real games, but in certain circumstances, a suicidal move may threaten the opponent's eye shape, yielding a ko threat.<ref name="Suicide in different rules.">{{cite web|title=Comparison of Some Go Rules|url=http://www.britgo.org/rules/compare.html#suic|publisher=British Go association|access-date=13 January 2017}}</ref>

=== 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 (go)|Komi]] (compensation for going second) also varies, ranging from several fixed values (commonly 5.5, 6.5, or 7.5) to various ''meta-games'' to determine a value (notably [[Komi (go)#Auction komi|Auction Komi]]).

=== Board sizes ===
Most [[Go (board game)|Go]] is played on a {{nowrap|19 × 19}} board, but {{nowrap|13 × 13}} and {{nowrap|9 × 9}} are also popular sizes. [[History of Go|Historically]] other board sizes were commonly used (notably {{nowrap|17 × 17}}, a predecessor of the {{nowrap|19 × 19}} board in ancient China). Go is also sometimes played on various ''novelty'' sized boards as small as {{nowrap|5 × 5}} and larger than {{nowrap|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).

=== Scoring ===
Historically in China a scoring system was used that penalized the player who had the greatest number of unconnected live groups of stones. On the basis that every group needs two eyes to be alive, and that the two eyes could not be filled in, two points were deducted from the score for each live group at the end of the game. This was known as the "cutting penalty" in Chinese, and is sometimes referred to as the "group tax" in English.<ref>{{citation|last=Fairbairn|first=John|title=New in Go|url=http://www.gogod.co.uk/NewInGo/NewInGo.htm|archive-url=https://web.archive.org/web/20130112213532/http://www.gogod.co.uk/NewInGo/C%26IP.htm|chapter=The rules debate as seen from Ancient China|chapter-url=http://www.gogod.co.uk/NewInGo/C&IP.htm|publisher=Games of Go on Disc|access-date=2009-06-09|archive-date=2013-01-12|url-status=dead}}</ref> This rule is not applied in modern Chinese scoring.

=== 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 whether draws (jigo) should 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 their stones, their 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 could be captured if all neutral territory 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.