Also 0-0. The move notation for kingside castling. (PGN format uses Os; FIDE uses 0s.)

In standard chess notation, kingside castling is notated O-O (or 0-0), while queenside castling is notated O-O-O (or 0-0-0), indicating the number of spaces the rook jumps.

Castling is a move in the game of chess involving a player's king and either of the player's original rooks. It is the only move in chess in which a player moves two pieces in the same move, and it is the only move aside from the knight's move where a piece can be said to "jump over" another.

Castling consists of moving the king two squares towards a rook on the player's first rank, then moving the rook to the square over which the king crossed.Castling may only be done if the king has never moved, the rook involved has never moved, the squares between the king and the rook involved are unoccupied, the king is not in check, and the king does not cross over or end on a square attacked by an enemy piece. Castling is one of the rules of chess and is technically a king move (Hooper & Whyld 1992:71).

The notation for castling, in both the descriptive and the algebraic systems, is 0-0 with the kingside rook and 0-0-0 with the queenside rook; in PGN, O-O and O-O-O are used instead. Castling on the kingside is sometimes called castling short (the rook moves a short distance, i.e. two squares) while castling on the queenside is called castling long (the rook moves a long distance, i.e. three squares) (Hooper & Whyld 1992).

Castling was added to European chess in the 14th or 15th century and did not develop into its present form until the 17th century. The Asian versions of chess do not have such a move.