Capture three to make an eye
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An example at the edge of the board.
This works away from the edge, too.
Capturing three White stones at makes an eye too, so Black lives.
... kills. Since a and b are miai, White dies.
Here is another counter-example, where the corridor has already been cut. Such counter-examples can also be found away from the side.
... recaptures. Because of and the eye around a is a false eye.
A possible heuristic to see whether the "capture three to make an eye" proverb applies is to look at the position immediately after capturing. In each of the four cases below, we see whether we can form an Black eye at either the a points. If White can occupy all the b points after Black's capture, then Black has no eye here.
- Capture Two Recapture One (a somewhat similar shape)
- The pages in the Eyes Collection, for other ways to make eyes
- Doug's Go Blog/Februari2004 (§ 8 March) for remarks on a case where the rule fails
PJT 2019-12-16: The Korean(?) text “석점을 잡으면 한 눈을 만들 수 있다” was substituted for the title of the first section. If this means the same, we might put it in a CJK box; if it uses a colourful image, it would be nice to know; if it was vandalism, we should lose it.