ko

A ko is a local board position in which one stone can be captured, but the capturing stone cannot be recaptured immediately because doing so would recreate the previous board position. After a ko capture, the opponent must play elsewhere before making the recapture.

The ko rule prevents endless cycles of captures and ensures that the game progresses. Ko is the basic repetition situation and rule. Superko is broader: under superko rules, a move that recreates any earlier position may also be prohibited. Double ko, triple ko, and other named ko forms describe more complex configurations. The exact restriction depends on the ruleset: some rules forbid only immediate repetition, while superko rules forbid repetition of any earlier position.