Retrograde analysis

Retrograde analysis starts from a position and asks about its past. Instead of calculating what may happen next, it uses the rules of chess to deduce what must have happened before, what could have happened, or what is impossible.

An orthodox position, meaning one that uses the normal rules of chess, is legal if it can be reached from the initial array by legal moves. From that requirement, a retro problem may ask what the last move was, which pieces were captured or promoted, whose turn it is, or whether special rights such as castling or an en passant capture can still exist given the previous play. Each conclusion must follow from verifiable constraints in the diagram.

A proof game is closely related: constructing a legal sequence to the diagram proves that route is possible. Retrograde analysis can also establish that certain histories are impossible or that a past event was necessary, which cannot be shown merely by finding one possible game.