Ko
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Table of contents | Table of diagrams Ko Ko at the side Another ko at the side Ko in the corner Not ko |
Ko is a Japanese go term adopted into English usage. It describes a situation where, without the ko rule, the game may not progress due to infinite repetitions.
The ko and the ko rule
Consider the situation shown in the diagram, and suppose no ko rule exists. For the position shown on the left, Black can capture the stone at a, resulting in the position shown on the right. Then White can immediately recaptures the
stone at b, reverting to the position shown on the left. If neither player gives way, then we have Black a, White b, Black a, White b, ..., repeated ad infinitum, stalling the progress of the game. This situation is known as a ko. To avoid the infinite sequence, the ko rule is introduced:
- If one player takes the ko, the opponent is prohibited from recapturing the ko 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.
Kos need not occur only in the center of the board. They can also show up at the sides or corners of the board, as shown in the diagrams below.
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. This situation is known as capture two, recapture one.
Ko in different rulesets
The ko rule as explained here is the standard ko rule, but there are also more general 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.
Further reading
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
- Never make hollow ko threats
More ko pages:
- For beginners: Basics on kos/Example
- Ko Types
- Double Ko, Triple Ko, Quadruple Ko, Multiple Ko
- Hanami-ko
- Moonshine Life
- Half-point ko (minimal ko)
- Real half point ko
- Superko
- Fighting Ko
- Disturbing Ko
- Ko threat and Ko Fighting
- Ko Threat Evaluation
- Ko Rules and Rules of Go
- Mathematical Ko Terms
- Ko Master
- Ko Loser
- Ko Monster
- Game Deciding Ko
- Ko Terminology
- Smallest Kos
- Unremovable Ko
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.
[2] sam: Isn't it easier to say "Ko is a rule that says that you cannot exactly recreate the board as it was at the end of your last turn." ? That both defines the ko situation, and the rule. What we have now doesn't make sense, unless you know what a Ko is, and if you do then you don't need the definition given?
anonymous: It is correct that in Japanese this is written with katakana, not hiragana?
anon: Yes.
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