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A group with five vacant "Me," as shown in the preceding diagrams, is a situation well known to the Japanese, so much so that they have a special phrase or saying that applies to it, to wit, "Go moku naka de wa ju san te," which means that it takes thirteen turns to reduce a group having five such "Me" in the center.
As we have previously seen, in actual play this white group would be regarded as "dead" as distinguished from "taken," and this series of moves would not be played out. White obviously would not play in the space, and he could not demand that Black play therein in order to complete the actual surrounding of the stones, and the only purpose of giving this series of diagrams is to show theoretically how the white stones can be killed. However, the killing of these stones would be necessary if the surrounding black line were in turn attacked ("Semeai"), in which case it might be a race to see whether the internal white stones could be completely surrounded and killed before the external white group could get in complete contact with the black line.
Stones which are sacrificed in order to kill a larger group are called "Sute ishi" by the Japanese, from "Suteru," meaning "to cast or throw away," and "Ishi," a "stone."
It may be noted that if a group contains four connected vacant intersections in a line it is safe, because if the adversary attempts to reduce it, two disconnected "Me" can be formed in the space by simply playing a stone adjacent to the adversary's stone, as shown in Plate 5, Diagram m, where, if Black plays for instance at K 11, White replies at L 11, and secures the two "Me." Even if these four connected vacant intersections are not in a straight line, thev are nevertheless sufficient for the purpose, provided the fourth "Me" is connected at the end of the three, and the Japanese express this by their saying "Magari shimoku wa me," or four "Me" turning a corner. Neither does it make any difference whether the four connected "Me" are in the center of the board or along the edge. On Plate 5, Diagrams iv and v, are examples of "Magari shimoku wa me," and they both are safe. It is interesting, however, to compare these situations with that shown at Plate 4, Diagram II, where the fourth intersection is not connected at the end of the line, and which group Black can kill if it is his move, as we already have seen.
If, however, such a group contains only three connected vacant intersections, and it is the adversary's move, it can be killed, because the adversary by playing on the middle intersection can prevent the formation of two disconnected "Me." We saw a group of this kind on Plate 2, Diagram IX, which can be killed by playing at N1l. Obviously, if it is Black's move in this case, the group can be saved by playing at Nn; obviously, also, if White, being a mere novice, plays elsewhere than at N 11, Black saves the stones by playing there and killing the white stone. Plate 5, Diagram VI, shows another group containing only three vacant intersections. These can be killed if it is Black's move by playing at A 1. On the other hand, if it is White's move, he can save them by playing on the same point.
Of course, if a group of stones contains a large number of vacant intersections, it is perfectly safe unless the vacant space is so large that the adversary can have a chance of forming an entire new living group of stones therein.
We now come to the one exception to the rule that the 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 1. 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 shov^n 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.