Ko
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The ko rule
The situation in this diagram is called a ko, a Japanese go term adopted into English. 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 at a, White at marked stone, and so on to infinity. To avoid this, there is a basic 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 elsewhere [1]. If White takes interest in the ko, his play elsewhere is called a ko-threat. If Black answers, White 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 retaking the ko, until either player decides to resolve the ko or that plays elsewhere are bigger. Such a sequence is called a ko fight.
Not all capture-recapture positions are ko
Note that a capture-and-recapture position is only ko if there is one stone captured and one stone recaptured. If Black captures 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.
Ko in different rulesets
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, depending on the governing Rule set for the game. 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 often do not know it.
Proverbs:
- If you don't like ko, don't play go
- The carpenter's square becomes ko
- There are no ko threats in the opening
See also:
- For beginners: Basics of kos
- Double Ko, Triple Ko, Multiple Ko
- Other repeating positions
- Hanami-ko
- Ko threat and Ko Fighting
- Moonshine Life
- Half-point ko (minimal ko)
- Real half point ko
- Ko Rules and Rules of Go
- Ko Master
- Ko Loser
- Ko Monster
- GameDecidingKo
Ko etymology
Unresolved. See Ko etymology discussion.
Famous games involving ko
Go Seigen - Iwamoto Kaoru, 1948
[1] ... or pass, which would only happen at the end of the game, when no interesting moves are available.
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