Ko - Introductory
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A ko is a position, where a single stone in atari can be captured, but the capturing stone will end in atari as well. Without a ko rule this can lead to infinite repetitions of that board position due to repeated capturing. A specific ko is shown in diagram (a). Assuming Black plays first, then can capture a White stone in diagram (b), leading to the position in diagram (c). Without the ko rule, can recapture the Black stone (diagram (d)) and revert back to diagram (a). If both players keep playing moves and , then the game will not terminate due to infinite repetitions.
To avoid this situation, the ko rule is introduced: White is prohibited from recapturing the ko at the turn immediately after has captured the ko. Therefore, has to play elsewhere, and if does not end the ko, then can recapture the ko. Of course, the ko rule applies to Black as well, in the sense that when White manages to capture the ko, Black cannot recapture the ko immediately.
A ko need not necessarily appear only in the center. The diagrams on the left show that kos can also appear at the side or the corner. In each case, the ko is indicated by and .
A ko-threat is an intermediary play outside the ko. Your opponent may answer the threat or resolve the ko. If he answers, then you may recapture the ko. If he resolves the ko, then you can fulfil your threat.
In the example answering the threat (b diagram) is correct, if White ignores (c diagram) to resolve the ko, captures three stones (and kills all White stones on the board - d diagram)