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I am trying to get a better understanding of the basic rules of Go. Assume that black has surrounded a white group of stones which has exactly one eye (liberty) inside the group.

What would happen if black placed a stone into this eye? All the white stones will be captured, but what happens to the black one placed into the eye?

I guess my question revolves around the question how capturing is checked after a move. Is it the non-active player's stones first and then the active player's or both at the same time?

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    A minor point, but the term liberty is preferable, as eye is also used to refer to a connected set of internal liberties, see eye and eyespace on Sensei’s Library. In fact a satisfactory formal definition of an eye is surprisingly hard to achieve! – PJTraill Apr 4 '18 at 17:27
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This is confusing to many new players. The rule is:

  1. Always capture opponent stones first. That means all opponent groups which are without liberties.
  2. Then, and only then, see if your own stone, or more precisely the group it belongs to, is without liberties

If (after capturing) your own stone is without liberties, the result depends on the rule set used:

  • In Japanese and Chinese rules, your move was simply illegal. So don't do this!
  • In New Zealand rules, your own stone (group) now gets captured.

For the second point, you may be wondering why one would ever intentionally play a stone that is immediately taken from the board. The simple answer is that this can sometimes be a ko threat. It can rarely also influence a semeai (capturing race).

There are a few details when performing suicide of a single stone regarding ko/superko, but you would likely never encounter it in practice.

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