O-O-O

O-O-O represents castling on the queenside. In this move, the king shifts two squares toward the rook, and the rook finishes immediately on the king's other side. In the standard starting arrangement, the rook begins on a1 for White and a8 for Black: the White king goes from e1 to c1 and the rook from a1 to d1; for Black, the king goes from e8 to c8 and the rook from a8 to d8.

The general castling conditions apply: the king and rook have not moved, the path between them is empty, the king is not in check, and the king's starting, crossing, and destination squares are not attacked. On this side, b1 or b8 must also be empty because it lies between the king and rook. It may nevertheless be attacked because the king never crosses that square.

Standard Algebraic Notation (SAN) and Portable Game Notation (PGN), the plain-text format for exchanging games, use three uppercase letters O: O-O-O. Tournament scoresheets governed by the International Chess Federation (FIDE) use 0-0-0 with zeroes. Both forms therefore circulate, but PGN requires O-O-O. Queenside names the half of the board; the queen does not take part in the move.