Reversible move

In chess programming and rules analysis, a reversible move usually means a king, queen, rook, bishop, or knight move to an empty square. It is neither a pawn move nor a capture, the two events that reset the count used for the fifty-move rule.

Programs often store a halfmove clock, where a halfmove is one side's turn. A reversible move increases this counter; a capture or pawn move resets it to zero. When each player has completed fifty consecutive moves without either event, a right to claim a draw may exist.

The name does not guarantee that every consequence can be undone. A king or rook moving from its original square may permanently lose the right to castle, the special move that transfers the king together with a rook, even if the piece later returns. A visually repeated arrangement therefore may not be the same position for the repetition rule.

Outside technical contexts, reversible may also be used informally for a piece move that could be taken back later. The context matters: the fifty-move classification is broader than the literal idea of restoring every positional right.