Add a second stone and sacrifice both

    Keywords: Proverb

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

Main discussion

Consider the following sequence (discussed by Sakata Eio in his book "The Middle Game of Go").

[Diagram]
Crosscut  

Black has just played the crosscut at B1. (Sakata calls B1 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.)

[Diagram]
Add a second stone  

It is clear that black+circle is going to be captured. Nevertheless Black adds another stone, in accordance with the proverb.

[Diagram]
Kikashi  

The thinking behind Black's line of play becomes clear in this diagram. Black has forcing moves at B1, B3 and B5. Thanks to his larger sacrifice, he has a move on the outside (B1) and two which seal off the corner. (Note that B5 makes a a gote follow-up for White. See /continuation for further explanation.)

[Diagram]
Direct atari: not so good  

Compare this with an atari at B1 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.

[Diagram]
Direct block: also not good  

If B1 simply blocks, W2 will capture and the atari at a is no longer there.

[Diagram]
Summary  

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

[Diagram]
Nearby stones  

For the sake of discussion, if there is a white+circle stone, then White will almost always capture black+circle by W1 (ladder), and almost never play W1 at a. This leaves only the forcing move at b, but the block at c is not sente.



Add a second stone and sacrifice both last edited by tapir on September 28, 2015 - 15:34
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