Each move in a game of chess consists of the displacement of one piece only, with the exception of what is termed "castling," in which the King and either Rook can be moved simultaneously by either player once in a game. In castling, the King moves sideways to the next square but one, and the Rook to which the King is moved is placed on the square which the King has skipped over. Castling is only allowed if neither the King nor the Rook concerned have moved before, and if there is no piece between the Rook and King.