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Ko
  Difficulty: Beginner   Keywords: Ko, Go term

Japanese: コゥ

[Diagram]
Diag.: ko

The situation in this diagram is called a ko. A special rule exists for this type of situation, to avoid infinite repetitions. Suppose that, without a ko rule, black were to play in this position. Black could capture one stone at A, after which white could recapture at the place where the marked stone stands, black A, white marked stone, and so on to infinity. To avoid this, there is a ko rule:

  • In a ko situation, if one player takes the ko, the opponent may not recapture immediately.

So if in this position black captures at A, white may not play immediately at the marked point, and instead will have to play somewhere else. She could use this move to play a so-called 'ko threat' - for example, threatening to capture a lot of stones. If black answers, she may now retake the ko, and it is black who is not allowed to take back immediately. This way the game might go on for a while by each player in turn making a ko threat and re-taking the ko. Such a sequence is called a ko fight.


[Diagram]
Diag.: Not ko

Note that a capture-and-recapture position is only ko if there is one stone captured and one stone recaptured. If black would capture at A in one of the situations above, white is allowed to retake at the position of the marked stone. This does not cause any infinite loops, and is not disallowed by the ko rule.

The ko rule as explained here is the standard ko rule, but there are also more complicated rules possible, which also disallow longer sequences of repetitions. The SuperKo rule simply states that any repetition of a previous situation is disallowed. The Ing ko rule is rather complicated, and even those who play in tournaments played according to the Ing rules do often not know it.



See also:



This is a copy of the living page "Ko" at Sensei's Library.
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