Castling notation

Castling moves the king and one rook as a single move, so it receives a special notation rather than two separate move records. In PGN, kingside castling is written O-O, and queenside castling is written O-O-O.

The letters do not name squares or count pieces. They are a visual convention. In standard chess, O-O leaves the king on the g-file and the rook on the f-file; O-O-O leaves the king on the c-file and the rook on the d-file.

Capital O or zero

The convention depends on the setting. The PGN specification requires capital letter O, while the notation appendix in the FIDE Laws prints 0-0 and 0-0-0 with zeroes. They describe the same two moves, but a scoresheet, book, or program may expect one form.

A check or checkmate marker can follow the castling notation, as in O-O+ or O-O-O#. The suffix describes the result of the move; it does not alter the meaning of castling.