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OldTwoStepKo

 

Approach Ko
   

Authors: Morten Pahle, BillSpight
Wiki-editor: Dieter Verhofstadt
Original discussion at: OldTwoStepKo

Approach ko

A ko is called an approach ko for White (Black) if White (Black) cannot win the ko without making an approach move. The Japanese term for it is yoseko.


[Diagram]
Diag.: Approach 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.



Going on, if black would have 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 will have to win it. A two-move approach ko or 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 ignore 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.



This is a copy of the living page "Approach Ko" at Sensei's Library.
(C) the Authors, published under the OpenContent License V1.0.