Castling rights

Castling rights record part of a position’s history. Castling is the special move in which the king moves two squares toward a rook and that rook moves to the other side of the king. At the start of a game, each player has one right on the side where the king begins and one on the side where the queen begins.

A right is permanently lost once the king has moved, once the corresponding rook has moved, or when that rook is captured on its original square. Returning either piece to its starting square does not restore the right. This is why the visible placement of the pieces alone does not always show whether castling remains available.

A retained right can still be unusable now

A player may retain the right but be unable to castle on the current move. Castling is temporarily prevented when pieces stand between the king and rook, when the king is in check, meaning under immediate attack, or when the king would cross or finish on an attacked square, meaning a square an enemy piece could capture on. Those obstacles may disappear later; a lost right does not return.

Castling rights also matter when positions are compared for a repetition draw claim, which allows a player to claim a draw when the same position appears for the third time. Two boards with identical pieces on identical squares are not the same legal position if castling is still possible in one but not the other.