castling

Castling is a special move in chess that involves both the king and one of the rooks. It's the only time a player can move two pieces in a single turn.1

To castle, the king moves two squares towards a rook, and then the rook moves to the square the king crossed over. For example, a player might move their king from e1 to g1 and their rook from h1 to f1.1

Castling can only be done once per game. It's not allowed if the king or rook has moved previously, if any squares between them are occupied, or if the king is in check or would move through or land on a square attacked by an enemy piece. In freestyle chess, the final positions of the king and rook are the same as in regular chess, regardless of their starting squares. The verb "castle" can also mean to move the king in this way.123