Set play

Set play is a virtual phase that already exists in the initial position. In a directmate, where White must force mate while Black defends, it is found by imagining that White could miss a turn and allowing Black to move first. If certain Black defenses already have prepared White replies, those variations form the set play.

That imagined pass is not a legal chess move. It is a way to describe content present before the key, the correct first move that begins the actual play. Comparing what was prepared beforehand with what happens afterward can reveal thematic changes in replies or mating moves.

In a helpmate, the term is used somewhat differently: set play can be an additional solution beginning with the side that would normally move second, usually White. Together with try play and actual play, set play can form an important part of the complete solution.