In chess, a cross-check is a tactic that is a check played in reply to a check, especially when the original check is blocked by a piece which itself either delivers check or reveals a discovered check from another piece. Sometimes the term is extended to cover cases in which the king moves out of check and reveals a discovered check from another piece (this is also known as a royal check); it does not generally apply to cases where the original checking piece is captured.
The cross-check is an essential tactic in winning some endgames such as with two queens versus one or a queen and pawn versus a queen. In these, the defense usually tries for a perpetual check and sometimes the stronger side can stop it only by a cross-check.
Cross-checks are rare in actual play, though they do occur, and in some endgames, particularly queen endgames, they are very important. The position shown to the is the final position in the famous queen endgame from the game between Mikhail Botvinnik and Nikolay Minev, Amsterdam Olympiad, 1954. In the position shown, after 91. Kc5!!, Black resigned because the promising looking checks 91...Qc7+, 91...Qg1+, 91...Qf2+ and 91...Qc2+ are answered by the cross-checks 92.Qc6+, 92.Qd4+, 92.Qd4+ and 92.Qc4+ respectively, forcing an exchange of queens in all cases, which will result in the promotion of the pawn and winning the game by a basic checkmate. This is an important theme in queen endgames: the weaker side often gives a series of checks, and it is frequently important for them to avoid simplifying cross-checks such as these in reply , . If the black queen attacks the pawn instead of checking it does no good since the promotion square of the pawn is protected by its queen.
Cross-checks are relatively infrequent in actual play, but are popular in chess problems since they make for a relatively unobvious solution. In the problem shown to the right, white is to move and mate in two moves against any defence. It is by G.F. Anderson and was first published in Il Secolo in 1919. The key 1.Kd6, threatening 2.Qb7#, gives the black king two flight squares, and two of the variations exploit these: 1...Kb6 2.Bc2# and 1...Kb4 2.Kxc6#. The other two defences exploit the fact that the key exposes the white king to checks, and are answered by cross-checks: 1...Rg6+ 2.Be6# and 1...Rd3+ 2.Bd5#.