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Internal ko-threat
Difficulty: Beginner
Keywords: Ko
An internal ko threat is a threat that cannot be ignored, because ignoring it makes winning the ko useless. This kind of threat should therefore be played before any other kind.
In this diagram, black is playing ko for the life of his group in the corner. Black has just captured the ko with the marked stone. The point a is now an internal threat for white. If black ignores this and connects the ko, white will play b, killing the black group. So black must play at b himself to avoid this. White can now retake the ko. Charles Matthews I prefer 'local ko threat' - there are certainly cases in capturing races where internal ko threats would be external, which is pretty confusing. As far as order of playing threats is concerned, such threats behave somewhat like threats of very high value (say 250). It isn't true that you ought to play high-value threats first. One of the reasons for that is that you prefer to save them for a later, more important ko. That reason is no good as applied to local threats - you could put it the other way round, that you prefer to finish all the local threats you have before the ko is over. There is another reason for delaying high-value threats, which is that they may be loss-making threats. Now this certainly may also apply to local threats. They can lose points, either for sure, or if you are anyway going to lose the ko and they make the loss worse. They may also be aji keshi, or throw away liberties or create cutting points, or just in some way have an adverse effect that has to be balanced against their possible gainful effect on the ko fight. I can certainly show an example from a pro game where one side delays using local threats until the final stages of a ko. I think you can explain that as a probing mechanism: you can be completely sure that local threats enable you to retake the ko. You have that information, you in some way want to test the opponent's approach to fighting the ko (for example, will she start using loss-making threats?). This seems to be a novel theme in analysis of ko fights: using local threats is a kind of forcing play, holding them back can allow you time to simplify some issues. Sorry if this seems complex ... ko just isn't that easy. This is a copy of the living page "Internal ko-threat" at Sensei's Library. (C) the Authors, published under the OpenContent License V1.0. |