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.

Exact return possibleCount not reset with a right lost

The legal state returns while the clock advances

8

7

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a1

b

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Start

Na3

Nh6

Nb1

Ng8

Initial clock at seven

White is to move, there are no castling rights or en passant capture, and the halfmove clock is seven.

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.