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Old Two Step Ko
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 if white wins 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. This is called a two step ko. A 'normal' ko is called a direct ko. Bill Spight: The proper term for this kind of ko is approach ko (yoseko). White cannot win the ko without making an approach move. Properly speaking, in a two-step ko taking the ko produces another ko mouth, which must be taken before winning the ko. Example:
Going on, if black would have even more liberties we could also get a three step ko, a four step ko etcetera. Bill: Better say 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 will have to win it. A two or three step ko can be considered a reasonable fight, although worse than a direct ko of course, but a five or six step ko is rarely worth fighting. Bill: 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. Bill: The size of the ko threats should increase after each approach move. With correct play, this balances the effect of leaving ko threats unanswered. This is a copy of the living page "Old Two Step Ko" at Sensei's Library. (C) the Authors, published under the OpenContent License V1.0. |