A move in which the king and a rook are moved at the same time. It moves the king from the center to a flank where it usually is safer, and it develops the rook. It is the only time two pieces are moved in a turn. Castling can be done on either the kingside (notated 0-0) or the queenside (0-0-0). Castling cannot be done in reply to a check, nor if either the king or rook has already moved.