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[[Image:Go ko.png.jpg|thumb|right|An example of a situation in which the ko rule applies.]]

[[Image:Go ko.png.jpg|thumb|right|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 position before the opponent's last previous move. This rule, called the ''[[Rules of Go#Ko (no repetition of the same shape)|ko rule]]'', prevents unending repetition.<ref>{{cite web | url = http://senseis.xmp.net/?Ko | title = Ko | author = Sensei's Library | accessdate = 2007-12-20 }}</ref> 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 would result in an unending cycle of captures by both players. The ko rule therefore prohibits White from playing at the marked intersection. 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, she will try to find a play that Black must answer; if Black answers, then White can retake the ko. A repetition of such exchanges is called a ''ko fight''.

Players are not allowed to make a move that returns the game to the position before the opponent's last previous move. This rule, called the ''[[Rules of Go#Ko (no repetition of the same shape)|ko rule]]'', prevents unending repetition. 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 would result in an unending cycle of captures by both players. The ko rule therefore prohibits White from playing at the marked intersection. 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, she will try to find a play that Black must answer; if Black answers, then White can retake the ko. A repetition of such exchanges is called a ''ko fight''.

Instead of placing a stone, a player may ''pass'', indicating that he sees no way to increase his territory or reduce his opponent's territory. When both players pass consecutively, the game ends and is then [[Rules of Go#End|scored]].

Instead of placing a stone, a player may ''pass'', indicating that he sees no way to increase his territory or reduce his opponent's territory. When both players pass consecutively, the game ends and is then [[Rules of Go#End|scored]].

=== Scoring Rules ===

=== Scoring Rules ===

There are two basic ways to count the score at the end of the game. The oldest of these is called ''territory scoring'' and is used in Japan, Korea and most Western nations. The other scoring rules &mdash;invented in 15th-century China<ref>{{citation | title = New in Go | chapter = The Rules Debate | author = John Fairbairn | url = http://www.gogod.co.uk/NewInGo/NewInGo.htm | chapterurl = http://www.gogod.co.uk/NewInGo/C&IP.htm | accessdate = 2007-11-27}}</ref>&mdash; are called ''area scoring'' and are used in China and some other countries. The US uses a scoring system that is a hybrid of these two systems.<ref name=AGAConcise>{{cite web | url = http://www.cs.cmu.edu/~wjh/go/rules/AGA.concise.html | title = AGA Concise Rules of Go | author = AGA Rules Committee | year = 1991 | accessdate = 2007-11-30}}</ref>

There are two basic ways to count the score at the end of the game. The oldest of these is called ''territory scoring'' and is used in Japan, Korea and most Western nations. The other scoring rules &mdash;invented in 15th-century China &mdash; are called ''area scoring'' and are used in China and some other countries. The US uses a scoring system that is a hybrid of these two systems.

With territory scoring, a player's score is the number of empty points enclosed by his stones, plus the number of prisoners he has. Prisoners are opposing stones which he has captured during the game or those that are still on the board but cannot avoid capture.

With territory scoring, a player's score is the number of empty points enclosed by his stones, plus the number of prisoners he has. Prisoners are opposing stones which he has captured during the game or those that are still on the board but cannot avoid capture.

With area scoring, a player's score is the number of stones he has on the board, plus the number of empty intersections he has surrounded. Prisoners do not count toward the score under these rules.

With area scoring, a player's score is the number of stones he has on the board, plus the number of empty intersections he has surrounded. Prisoners do not count toward the score under these rules.

Given the fact that the number of stones a player has on the board is directly related to the number of prisoners his opponent has taken, the resulting score under both rules is often identical and is rarely more than one point different<ref>{{cite web | url = http://www.cs.cmu.edu/~wjh/go/rules/AGA.commentary.html | title = Demonstration of the Relationship of Area and Territory Scoring | author = Fred Hansen}}</ref>. Under US rules, players hand over a prisoner when they pass, called a ''pass stone''<ref name=AGAConcise/>. Combined with the requirement that White makes the last pass, this system guarantees that the ''net'' score under territory scoring will be the same as that under area scoring.

Given the fact that the number of stones a player has on the board is directly related to the number of prisoners his opponent has taken, the resulting score under both rules is often identical and is rarely more than one point different. Under US rules, players hand over a prisoner when they pass, called a ''pass stone''. Combined with the requirement that White makes the last pass, this system guarantees that the ''net'' score under territory scoring will be the same as that under area scoring.

Each of these scoring methods has advantages and disadvantages.<ref>{{cite web | url = http://home.snafu.de/jasiek/advant.html | first = Robert | last = Jasiek | title = Objective Advantages of the Scoring Methods | year = 2007 | accessdate = 2007-11-30 }}</ref> There have been some efforts to agree on a standardized set of international rules.<ref>{{cite web | url = http://home.snafu.de/jasiek/int.html | author = Robert Jasiek | title = International Rules | accessdate = 2007-12-13 }}</ref>

Each of these scoring methods has advantages and disadvantages. There have been some efforts to agree on a standardized set of international rules.

=== Optional rules ===

=== Optional rules ===

Optional Go rules may set the following:

Optional Go rules may set the following:

* compensation points, almost always for the second player, see ''[[komidashi|komi]]''; Tournaments commonly use a compensation in the range of 5-8 points<ref name=histKomi>{{cite web | url = http://senseis.xmp.net/?HistoryOfKomi | author = Sensei's Library | title = History of Komi | accessdate = 2007-11-20 }}</ref>, generally including a half-point to prevent draws.

* compensation points, almost always for the second player, see ''[[komidashi|komi]]''; Tournaments commonly use a compensation in the range of 5-8 points, 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);

* compensation stones placed on the board before alternate play, allowing players of different strengths to play competitively (see [[Go handicap]] for more information);

* ''superko'': Although the basic ko rule described above covers over 95% of all cycles occurring in games<ref>{{cite web | title = Ko Rules | url = http://home.snafu.de/jasiek/korules.html | first = Robert | last = Jasiek | year = 2001 | accessdate = 2007-11-30 }}</ref>, there are some complex situations &mdash;''triple ko'', ''eternal life'', etc.&mdash; 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 ''any'' previous position. This is called ''superko'' <ref name="superko">{{cite web| url = http://senseis.xmp.net/?superko | author = Sensei's Library | title = Superko | accessdate = 2007-02-24}}</ref>.

* ''superko'': Although the basic ko rule described above covers over 95% of all cycles occurring in games, there are some complex situations &mdash;''triple ko'', ''eternal life'', etc.&mdash; 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 ''any'' previous position. This is called ''superko''.

== External Links ==

== External Links ==