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The exchange (chess)
The exchange in chess refers to a situation in which one player loses a minor piece (i.e. a bishop or knight) but captures the opponent's rook. The side which wins the rook is said to have ''won the exchange'', while the other player has ''lost the exchange'', since the rook is usually more valuable. Alternatively, the side that has won the rook is ''up the exchange'', and the other player is ''down the exchange''. The opposing captures often happen on consecutive moves, although this is not strictly necessary. It is generally detrimental to lose the exchange, although occasionally one may find reason to purposely do so; the result is an ''exchange sacrifice'' (see below). The ''minor exchange'' is an uncommon term for the exchange of a bishop and knight.
Note that ''the'' exchange differs from the more general "exchange" or "an exchange", which refers to the loss and subsequent gain of arbitrary pieces, for example to "exchange queens" would mean that each side's queen is captured.
==Value of the exchange==
The value of the exchange (i.e. the difference between a rook and a minor piece) has been considered for decades. Siegbert Tarrasch put its value as 1½ pawns in the endgame, but not for the opening or the first part of the middlegame. That is widely accepted today, but Jacob Sarratt, Howard Staunton, and José Capablanca felt that the exchange was worth two pawns. Tigran Petrosian thought that one pawn was the right value. Wilhelm Steinitz said that a rook is slightly better than a knight and two pawns but slightly worse than a bishop and two pawns . Cecil Purdy said that the value depends on the total number of pawns on the board. The reason is that when there are many pawns, the rooks will have limited mobility because there will not be open files. The exchange is barely worth 1½ points when there are 14 or more pawns on the board. Only when there are ten or fewer pawns may the exchange be worth 2 points . Purdy gave the value as 1½ points in the opening and increasing to 2 points in the endgame. In the middlegame the value would be closer to 1½ than to 2 . Edmar Mednis gave the value as 1½ in the endgame , . Max Euwe put the value at 1½ in the middlegame and said that two pawns are more than sufficient compensation for the exchange . Larry Kaufman's computer research puts the value as 1¾ pawns, but only 1¼ pawns if the player with the minor piece has the bishop pair . Hans Berliner puts the difference between a rook and knight as 1.9 pawns and the difference between a rook and a bishop as 1.77 pawns . In practice, one pawn may be sufficient compensation for the loss of the exchange, whereas two pawns almost always is .
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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