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Two Eyes
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  Difficulty: Introductory   Keywords: Life & Death

To be safe from capture, a group of stones must in general have two eyes [1]. This is the primary way to make groups alive (the other, usually less desirable way, is seki).

The basic rules of go state that when a group's last liberty is removed, the group is captured. It follows that if a group has at least two liberties which cannot be removed, the group cannot be killed and is alive. All subtleties aside, such an unremovable liberty is what we call an eye.

[Diagram]
Two Eyes

Look at the circled points. A white stone placed on any of them would be disconnected from the other white stones and would have no liberties. Both black chains would still have the other circled point as a liberty. Such a move by White would thus be illegal. Neither of the two circled points can be occupied by White without the other. In conclusion, the circled points are two unremovable liberties of the black group, or two eyes, and it is impossible to remove the black group from the board (without Black stupidly filling one eye).




Authors

  • Morten Pahle
  • StevertIgo
  • Dieter: Rather than a WME, I completely revised this page. There were both incorrect statements and too technical comments on it for a basic explanation. I avoided technical issues by including "generally speaking". Check out the [ext] previous version and recover what was left out unjustified.

[1] A group is generally considered to be alive if it is able to make two eyes in response to any threat.

Robert Pauli: Honestly, Bill, this footnote style doesn't attract me much (and lost the seki case, BTW). Shouldn't it better (and without footnote) be something like

A group of stones is safe from capture (is alive) if it can not be prevented to reach seki or (more desirable) to form two eyes.

Crystal clear and to the point, not?

Bill: Charles Matthews prefers the term, safe, to indicate stones that are alive without defense. Not everybody uses it that way, but, since there is a reasonable (and traditional) alternative, alive, for stones that may require defense, I am happy to stick with the traditional term, and not equate the two.

Since this is introductory material, I agree that it is best not to get too technical. If it were about life, like the alive page, I would agree that a discussion of seki belongs here. But it is not.

Logically, your sentence is clear. Operationally, it gets into areas that are beyond the ken of novices. How do you reach seki or form two eyes? And it is not really to the point of this page, but of the alive page.



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This is a copy of the living page "Two Eyes" at Sensei's Library.
(OC) 2004 the Authors, published under the OpenContent License V1.0.