Add a second stone and sacrifice both
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The proverb add a second stone and sacrifice both summarizes a common technique of sacrifice.
Table of contents | Table of diagrams Crosscut Add a second stone Kikashi Direct atari: not so good Direct block: also not good Summary Nearby stones Options Kikashi Gote Gote Tenuki |
Main discussion
Consider the following sequence (discussed by Sakata Eio in his book "The Middle Game of Go").
Black has just played the crosscut at . Sakata calls
vulgar style in the context of the discussed game, but that is another matter. There are plenty of occasions where this sequence is perfectly OK.
It is clear that is going to be captured. Nevertheless Black adds another stone, in accordance with the proverb. White answers at
to maintain the kill of these cutting stones captured.
The thinking behind Black's line of play becomes clear in this diagram. Black has forcing moves at ,
and
. Thanks to his larger sacrifice, he has a move on the outside (
) and two which seal off the corner (
and
, the latter making a a gote follow-up for White).
Compare this with an atari at without adding the extra stone: White would capture (a ponnuki), and that would be the end of it. The block at a is no longer sente now.
In summary: adding the extra stone increases the (marked) liberties of the black group. As White needs more moves to capture it, Black gets more forcing moves.
The effect of nearby stones
For the sake of discussion, if there is a stone, then White will almost always capture
by
(ladder), and almost never play
at a. This leaves only the aji at b, but the block at c is not sente.
More on the effect on thecorner
If Black at here,
can play tenuki.
ends in gote. Later there is the alphabetized endgame.
(Of course, if black is komaster, black could respond to white a with black c. But in general, black has to draw back to b because white will have little to lose in the ko.)
If Black does not play tenuki, he ends in gote. We can expect White a, Black b, White c in the late endgame.
(When compared with the sente diagram above, here is worth only about 9 points in gote. Before yose, this is relatively small, so tenuki is most likely a better option.)
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