players may place their stones at will on any vacant intersection on the board. This rule is called the rule of "Ko," and is shown on Plate 6, Diagram i. Assuming that it is White's turn to play, he can play at D 17 and take the black stone at C 17 which is already surrounded on three sides, and the position shown in Plate 6, Diagram ii, would then arise. It is now White's turn to play, and if he plays at C 13, the white stone which has just been put down will be likewise surrounded and could be at once taken from the board. Black, however, is not permitted to do this immediately, but must first play somewhere else, and this gives White the choice of filling up this space (C 13) and defending his stone, or of following his adversary to some other portion of the board. The reason for this rule in regard to "Ko" is very clear. If the players were permitted to take and retake the stones as shown in the diagram, the series of moves would be endless, and the game could never be finished. It is something like perpetual check in Chess, but the Japanese, in place of calling the game a draw, compel the second player to move elsewhere and thus allow the game to continue. In an actual game when a player is prevented from retaking a stone by the rule of "Ko," he always tries to play in some other portion of the board where he threatens a larger group of stones than is involved in the situation where "Ko" occurs, and thus often he can compel his adversary to follow him to this other part of the field, and then return to retake in "Ko." His adversary then will play in some part of the field, if possible, where another group can be threatened, and so on. Sometimes in a hotly contested game the battle will rage around a place where "Ko" occurs and the space will be taken and retaken several times.
