Approach Ko
A ko is called an approach ko for a player if that player must make an approach move in order to convert the ko into a direct ko. In the example below, the ko is a direct ko for Black and an approach ko for White because White will have to play at b before White can capture the four Black stones.
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.
Black needs 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, but as the saying goes, 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 count of an approach ko depends on who is its ko master. In this case it makes a difference of 5 3/4 points. See ko master for the calculation.
The exact phrase for the terms is not agreed upon. The following occur: "indirect ko with 1 approach move", "1-move approach ko", "1-step approach ko", "1-move approach move ko", "1-step approach move ko". Also see the links for yet further variation.
See also:
- X-ko (New terminology for approach kos)
- Rogue ko
- Direct ko
- Two-stage ko, Three-stage ko
- Two-step ko - Two-stage ko - Discussion
- /discussion
[1] The Japanese term for it is yose-ko; here yose indicates approach, not endgame.