A move by a King and Rook that serves to bring the king into safety and to activate the rook. The King is moved sideways two squares from its original square. At the same time, a Rook moves from its original square to the first square on the other side of the King. Castling can take place either to the queenside or to the kingside. It is the only way of moving two pieces in one turn. A player may only castle if both the king and rook have not moved before, his king is not in check, and his king does not pass a square on which it will be in check.