BQM 459

    Keywords: Question

CGT values of play

Was puzzeling about the following position

White's turn  

It is White's turn. Is there a difference between the CGT value of moves a and b? I am always wondering what is best.

Are they really equal?

Or am i just turning into [ext] Buridan's donkey

Can we add c to the comparison?

Tapir: There is a ko after b so they can't be really equal. The score difference between the 2 results for black / white winning the ko is 2 points. White playing a ends in the middle.

No ko (White at a)  

No ko and no ko threats remain in the corner.

White wins ko (one stone captured!)  

A single black ko threat remains in the corner at either a or 'b.

Black wins ko  

No ko threats remain in the corner.

White's turn  

fractic: The only 2 moves worth considering here are a and c. b offers no advantage over a. Because a leaves a ko threat playing c is advised early in the game but a is correct in the endgame. You might be interested in the discussion on do inferior plays have values.

Bill: Without seeing the surroundings we cannot say what is best. But let us assume that the left is White territory and the top is Black territory. That pretty well eliminates the solid connection. (However, it is usually best when the left is not solid territory for White.) You also have to take kos and ko threats into consideration.


W1 settles the corner. Black can cut at a as a ko threat.

Aji for Black  

The hanging connection does not necessarily settle the corner. For one thing, Black still has a ko threat at a.


B3 - W4 = ko threat and reply B5 takes ko back at 1

For another, under many circumstances Black can make a throw-in ko with B1. White's threat to capture black+circle is not as big as Black's threat to capture white+circle, so Black will often be able to force W6.

Good for Black  

That result is good for Black. We may assume that White can play W1 with sente. The net result is that White has lost one point of territory (W6 above) and Black has lost one stone (black+circle). However, Black can count 1/3 point for W1, which is in atari in the ko, so the net gain to Black by comparison is 1/3 point.

So sometimes the descent and hanging connection come to the same thing, but sometimes the hanging connection loses 1/3 point by comparison. The descent dominates.

BQM 459 last edited by Dieter on March 31, 2009 - 11:56
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