In chess, tempo refers to a turn or single move. When a player achieves a desired result in one fewer move, one gains a tempo and conversely when one takes one more move than necessary one loses a tempo. Similarly, when one forces one's opponent to make moves not according to the initial plan, one gains tempo because the opponent wastes moves. A move that gains a tempo is often called a move with tempo.
A simple example of losing a tempo may be moving a rook from the h1 square to h5 and from there to h8; simply moving from h1 to h8 would have achieved the same result with a tempo to spare. Such maneuvers do not always lose a tempo however. The rook on h5 may make some threat which needs to be responded to. In this case, since both players have "lost" a tempo, the net result in terms of time is nil, but the change brought about in the position may favor one player more than the other.
In the Scandinavian Defense, after 1. e4 d5 2.exd5 Qxd5, if White plays 3. Nc3 it attacks Black's queen, forcing it to move again, and White gains a tempo. A similar move gains a tempo in the Center Game opening.
In some endgame situations, a player must actually lose a tempo to make progress. For example, when the two kings stand in opposition (a form of zugzwang), the player to move is often at a disadvantage because he must move. The player to move may be able to triangulate in order to lose a tempo and return to the same position but with the opponent to move (and put him in zugzwang). Kings, queens, bishops, and rooks can lose a tempo; a knight cannot (Müller & Pajeken 2008:40,175,189).
In the above puzzle from a 2008 game between Artyom Timofeev and Ernesto Inarkiev, Black resigned because White will win with a tempo move. Timofeev won the 2008 Moscow Open with this game. White is threatening 118. Rh8+. If Black moves his king on move 117, White wins the bishop with 118. Rh8+, which results in a position which has an elementary checkmate. If Black moves 117... Bh5 then 118. Rh8 and Black is in zugzwang, and loses. So Black must move 117... Be2 to avoid immediately getting into a lost position. But then will come 118. Rh8+ Bh5 and now White makes a tempo move with 119. Rh7 (or 119. Rh6), maintaining the pin on the bishop, making it Black's turn to move, and Black must lose the bishop.
A spare tempo in an endgame is when a player has a pawn move that does not essentially change the position but it loses a tempo to put the opponent in zugzwang. In the above puzzle, if only the queenside pieces were considered, it would be an instance of reciprocal zugzwang, the player to move would lose. In the full position, White has two spare tempos (f2-f3 and h2-h3) whereas Black has only one (f7-f6), so White has a spare tempo. By using these moves he can force Black into a fatal zugzwang.
A pawn may have a reserve tempo, mainly in endgames involving only kings and pawns. This is especially true of a pawn on the second rank, where it has the option of moving one or two squares. Pawn moves held in reserve may be used to win a game. In the above puzzle from a 1986 game between John Nunn and Klaus Bischoff, Black resigned because he must lose his pawn on the d-file because White has a reserve tempo with his a-pawn.
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