Special Ko Rules when Determining Life and Death After Passing
During the normal course of play, the basic ko rule (which is universal across all go rules) is in effect:
Article 6. Ko
A shape in which the players can alternately capture and recapture one opposing stone is called a “ko.” A player whose stone has been captured in a ko cannot recapture in that ko on the next move.
When a ko arises that both players want to win (e.g., the life/death of a large group may depend on ko), they must fight for it by making ko threats elsewhere in order to retake it. Thus, the existence, size, and balance of ko threats across the whole board will determine who wins a ko fight and ultimately how key positions will be settled.
A classic example to illustrate this point is the “Bent Four in the Corner”. In particular, let’s consider the below shape, which is a temporary seki, where white could play at A in order to force black to capture 4 stones, but be left with the bent four in the corner shape with white to play.
From the bent four in the corner position, white can start a ko fight in an attempt to capture the black stones:
Whether this would be successful depends on the status of ko threats on the rest of board. Black may even have enough large unremovable ko threats, such that white would never be able to favorably win the ko threat. This would make black’s stones (in the first figure above) seemingly invincible, and white should not collapse the seki to start the ko fight.
However, in spite of this, black’s stones are still dead under Japanese rules.
White should never actually play at A in (the first figure), since the position will be ruled to be dead, regardless of what ko threats black has. This is the case, since the life and death confirmation phase introduces a special ko rule different than that used during normal play:
Article 7. Life and death
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2. In the confirmation of life and death after the game stops in Article 9, recapturing in the same ko is prohibited. A player whose stone has been captured in a ko may, however, capture in that ko again after passing once for that particular ko capture.
This rule has the effect of nullifying normal ko threats. Whether or not black has other ko threats becomes essentially irrelevant, since these threats would not allow black to retake the ko.
Note: the life and death confirmation phase are not actual additional plays, but rather the consideration of hypothetical, ideal play, just for the purpose of resolving life and death. For scoring, the board position, from immediately after the two passes, is restored, but with dead stones removed as prisoners (except for the case of anti-seki as discussed earlier).
Article 9. End of the game
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2. After stopping, the game ends through confirmation and agreement by the two players about the life and death of stones and territory. This is called “the end of the game.”
Article 10. Determining the result
1. After agreement that the game has ended, each player removes any opposing dead stones from his territory as is, and adds them to his prisoners.