Unlike chess, in which a board position can be repeated a number of times before a game is declared drawn, go does not allow a board situation to be repeated even once. The ko rule does not permit a board situation to be repeated. If a board situation is repeated during multiple ko situations, the game is immediately abandoned. As the rules of go generally do not permit draws, the game has to be played again.

4 simultaneous ko situations

In the last column, we looked at the first 100 moves of the game between O Meien (White) and Norimoto Yoda, the current Meijin. Black accumulated a lot of territory--he has all four corners, so White felt he must start making some himself. Both players had to think fast, because they were allotted only 10 seconds per move.

Diagram 1: (white 10 captures the ko at the marked stone)

White 2 builds territory at the top but his center stones are in danger. He tries to rectify this with 4. However, black sets up a ko with 7 and 9. After white 10 captures the ko, black 11 creates a lot of ko threats. White mistakenly believes he can ignore the threat, so he plays 12 and 14. This is another mistake as Black can cut with A, followed by white B and black C. White would have to fill in the ko (where black 5 was captured earlier), but then black D would bottle up 12 and 14. Black ignores White's mistake for the moment and kills White's entire group in the lower left. White's moaning could be heard on TV when he realized his mistake. However, he seeks revenge with 24 and 26. Black falls for it by playing at 27, leading to a double ko in the sequence through 35 (see Reference 1). White 36 puts a black stone in atari.

Diagram 2: (6, 11, 14, 29, 32, 35, 38, 43, 46, 61, 64, 69, 72, 77 capture the ko around the stone marked with a triangle; 12, 21, 30, 37, 44, 49, 54, 63, 70, 75 capture the ko around the stone marked with a cross; 20, 31, 36, 45, 48, 55, 62, 71, 74 capture the ko around the stone marked with a square; and 65, 68, 73, 76 capture the ko around 60)

The double ko in the lower left corner means White has an unlimited supply of ko threats and he cannot lose the ko in the center (the stone with the triangle). Occasionally, however, this ko is fought over to gain more time. As he is sure to win the center ko, White tries to turn the entire center into territory with 4. Remember, White only wins the center ko by virtue of the double ko in the lower left. Black invades with 15 and White with 16. Black ensures he will live in the sequence up to 47 while White seizes the upper right corner. However, black 53 through 60 start another ko. All ko situations are too important to lose so the players keep going back and forth with moves 68 through 77. Black 77 repeats the board situation and the game is declared forfeit.

Reference 1: This diagram gives a simplified picture of the real game. The lower left is a double ko, meaning that after black 1 captures A, he is basically safe. White can make a ko threat somewhere, then capture at A, but then Black plays at C to capture white B. Of course, White can continue playing ko threats and capture at B again, but then Black plays at 1 once more, etc. White cannot win the ko. However, he can use his captures at A or B as ko threats, and he never runs out of these threats. This is the reason he will win the center ko where the triangle-marked stone is.

Problems

Problem 1: This situation is part of a joseki. White can now play 1 and 3 hoping to fool Black. Black 4 and 6 is the correct response. Why?

Problem 2: What happens if Black plays at 4 and then captures at 6? Why are these moves poor?

Problem 3: Black just captured one stone with 1 threatening to kill the White stones. White can survive using a double ko by attacking the marked black stones. How?

Van Zeijst is a four-time European go champion and European representative at the Fujitsu World Championship.