Suicides are forbidden under most rulesets—but there are a few rulesets that do allow them.
For the vast majority of the cases, even if suicide is allowed, it only causes a loss of stones and/or points to oneself. However, in rare cases, suicide of more than one stone can be a useful move. Further on in this article there are examples of positions where suicide (if allowed) can be useful.
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(how about the aga, egf and other rule sets)
Suicide is normally forbidden under territory scoring, which makes sure the opponent can be eventually forced to pass - instead of continuously suiciding, which is never illegal under the simple ko rule that territory scoring uses. Area scoring, on the other hand, uses various other ko rules with broader effects (to fix its problems with prisoner-unbalanced repetition) - which also makes suicide potentially allowable.
(-- a bit about counting and suicide--)
A very unusual incident occurred in the first round of the 15th Korean Kiseong tournament (2003-10-02). Cho Hunhyun 9p was forced to lose a game (against Choi Cheolhan) because of an illegal move. You can see the game record at the time of the infraction at go4go. Cho obviously intended to take the ko at a in the lower left corner when he made an illegal one stone suicide play instead (on the lower edge). Such a mistake is extremely rare in professional tournaments.
For the vast majority of the cases, even if suicide is allowed, it only causes a loss of stones and/or points to oneself.
However, in rare cases, suicide of more than one stone can be a useful move. The rest of this article gives examples of how suicide can be useful.
Suicide as ko threat
The simplest, and best known, situation where suicide can make a difference is as a ko threat. In this position, the black group is alive. However, if White is allowed to play the suicide move of , taking three white stones off the board, Black will have to come back at the same point to make two eyes. (If black plays elsewhere to resolve the ko, then white can use their next turn to play at and kill the black group.) If suicide is forbidden, then White has no ko threat here.
But sometimes suicide also makes a difference for the outcome of a capturing race. In this diagram, if White is not allowed to play suicide, she is dead. There is no way for White to avoid that: Black first fills up two liberties with to , then captures at , resulting in the next diagram.
Playing at to prevent Black from getting two eyes will not help White.
Black plays atari at , and White is dead.
Note that these diagrams are just used to show that White is indeed dead - Black need not hurry to take the stones off the board.
...but, if suicides were permitted....
If suicide is allowed, White does have a resource in this situation. She can play the suicide move of . The result is shown in the next diagram.
After , the position is a seki; neither player can make any useful move - so this time, White lives. If Black plays or at or or a, the position is still seki, each player having one eye and no external liberties.
 The status of self-capture of a single stone in various rulesets which allow suicide is as follows:
- Ing rules explicitly forbid suicide of a single stone.
- Tromp-Taylor rules have the positional superko rule, so suicide of a single stone is not allowed.
- New Zealand rules have only situational superko, so suicide of a single stone is usually allowed. When allowed, it is equivalent to passing. It is not allowed after a pass.