Add a second stone and sacrifice both

    Keywords: Proverb

The proverb add a second stone and sacrifice both summarizes a common technique of sacrifice. Adding a stone has many possible effects:

  • increase liberties and require the opponent more moves to capture.
  • make opponent's eyespace less efficient.
  • induce opponent's capturing moves touching to the edge, thereby reducing liberties.
  • make later atari not a ko.

Table of contents Table of diagrams
Crosscut
Add a second stone
Kikashi
Direct atari: not so good
Direct block: also not good
Summary
Simple example

Increasing liberties

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.


Making eyespace less efficient

[Diagram]
Simple example  

The simplest example is this corner shape. If Black is allowed to play at 1, he secures two eyes very efficiently. White simply adds a stone to be captured. Two stone capture consumes more eyespace for making an eye.


Reducing opponent's liberties

See Two-stone edge squeeze and Two-stone corner squeeze.


Making later atari not a ko

See Gokyo Shumyo, Section 1, Problem 74 / Solution. White avoids a ko by adding a sacrifice stone (see the first Variation diagram).


Add a second stone and sacrifice both last edited by hnishy on July 11, 2018 - 15:01
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