Perpetual check

In the game of chess, perpetual check is a situation in which one player can force a draw by an unending series of checks. Such a situation typically arises when the player who is checking cannot deliver checkmate, while failing to continue the series of checks gives the opponent at least a chance to win. A draw by perpetual check is no longer one of the rules of chess. However, such a situation will eventually result in a draw by either threefold repetition or the fifty-move rule, but usually players agree to a draw (Burgess 2000:478).

Perpetual check can also occur in other chess variants, although the rules relating to it may be different. For example, giving perpetual check is not allowed (an automatic loss for the giver) in both shogi and xiangqi.

In this diagram, Black is ahead a rook, a bishop, and a pawn, which would normally be a decisive material advantage. But White, to move, can draw by perpetual check:

The same position will soon repeat for the third time and White can claim a draw by threefold repetition; or the players will agree to a draw.

In the second diagram, from Unzicker versus Averbakh, Interzonal 1952, Black (on move) would soon be forced to give up one of his rooks for White's c-pawn (to prevent it from promoting or to capture the promoted queen after promotion). He can, however, exploit the weakness of White's kingside pawn structure with

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