Illegal position

An illegal position is a placement and game state that cannot be reached from the starting position through legal moves. The definition considers every possible history, not merely whether the latest move looks unusual.

Clear examples include both kings being in check, meaning under immediate attack, at the same time, or an unpromoted pawn standing on the farthest rank, where it was required to become a queen, rook, bishop, or knight. By contrast, several queens do not make a position illegal, because pawn promotion can create extra pieces.

This is different from an illegal move. An illegal move is one action that breaks the rules; an illegal position is the resulting state or any other historically impossible state. When one appears during play through an irregularity, a departure from the correct state or procedure, the response depends on its cause. After a completed illegal move, normally after the clock is pressed, the previous position is restored and the relevant penalty may apply.