Approach Ko
A ko is called an approach ko for White (Black) if White (Black) cannot win the ko without making an approach move.
In this diagram, Black and White are involved in a ko fight around the position of the marked black stone and a. If Black wins this ko fight, he can capture the white stones and end the ko, but before White can win the ko fight, she will first have to fill up a liberty at b. Black can then re-take the ko, which will then be fought out the normal way. Thus, White will have to win this ko twice before getting what she wants. By way of contrast, Black can capture the White stones immediately. Black need to ignore at most one ko threat to capture White, whereas White may need to ignore one ko threat to play at a capturing Black's single stone, and then ignore another ko threat to play at b.
Going on, if Black had even more liberties we could also get a two-move approach ko, a three-move approach ko, etc.
White's chances of winning the ko do of course diminish the more often she would have to win it. A two-move approach ko can be considered a reasonable fight, although worse than a direct ko of course, but a five- or six-move approach ko is rarely worth fighting.
There is a saying, "A three-move approach ko is no ko."
White will have to ignore several black ko threats, which will probably cost her more than the ko will bring her. Alternatively, Black can play several ko threats and have them ignored; and then win the ko by ignoring only one ko threat himself.
The size of the ko threats should increase after each approach move. With correct play, this balances the effect of leaving ko threats unanswered.
The count of an approach ko depends on who is its KoMaster. In this case it makes a difference of 5 3/4 points. See KoMaster for the calculation.
Ko is an ambiguous term. It refers to one particular shape, a class of shapes, a particular nature of a fight, the fact that a ko shape is under fight, etc. The most frequent usage is for the 2 adjacent intersections and its basic ko shape. To reduce ambiguity, the nature of a ko should not also be called "ko". Therefore, a new term shall be introduced:
An x-ko is a ko with x approach-moves that one side has to make while for the other side the ko is direct.
So a 0-ko is what was called a direct ko. A 1-ko is an indirect ko with 1 approach move (also called 1-move approach ko or 1-move approach move ko). A 2-ko is an indirect ko with 2 approach moves (also called 2-move approach ko or 2-move approach move ko). Etc.
[1] The Japanese term for it is yose-ko; here "yose" indicates "approach", not endgame.
Authors: Morten Pahle, Bill Spight, RobertJasiek
Wiki-editor: Dieter Verhofstadt