!, 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.
Cross-checks are relatively infrequent in actual play, but are popular in chess problems since they make for a relatively unobvious solution.
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#.
The problem to the right, also by G. F. Anderson (first published in The Observer in 1961) and also a mate in two, features no fewer than five cross-checks.