Block problem

A block problem is usually a directmate, a composed position in which White must force mate against every defense. Its key, the unique first move that solves the problem, does not create a specific mating threat. Instead, it leaves Black compelled to move.

The resulting position is a zugzwang: Black has legal moves, but each one abandons a guard, blocks a square, opens a line, or causes another weakness. White exploits the particular consequence of that move to continue or mate.

Play prepared before the key

The diagram often contains set play, meaning replies that would work if White could skip the first turn. If every legal Black move already has a reply, the position is a complete block. The key must preserve that network or alter it deliberately. A block in which the key changes at least one prepared reply is called a mutate.