eye shape
A.k.a. gankei. A group of adjacent intersections which are either vacant or hold opponent stones that can be captured, from which one or more eyes can be made, forms an eye shape. Some shapes are guaranteed to form two or more eyes. Some are not (see nakade). For example, four stones in a square can make only one eye whereas, four in a line, if surrounded without defect, can make two. When evaluating a proponent eye shape, any defects where the opponent can force vacant intersections to be filled must be taken into account. In the first diagram it is Black's turn to play. Given that an eye at A is certain, and a second eye at B or C seems plausible, it might seem that the eye shape is sufficient for White to live. However, a sacrificial black stone played between A and B reduces that pair to a single eye shape. Here this leaves White the impossible task of making two moves to surround C without a spoiling reply in order to form the second eye. In the next diagram Black ignores the atari on the two black stones in favour of negating the eye at C (by filling the outside liberties, Black can force White to connect the three white stones at C). White captures the two black stones but (in the final diagram) Black throws in to make the captured space a false eye. |