O-O in Chess: Kingside Castling Notation

Explanation

O-O represents on the kingside. 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 h1 for White and h8 for Black: the White king goes from e1 to g1 and the rook from h1 to f1; for Black, the king goes from e8 to g8 and the rook from h8 to f8. The whole action counts as one move.

When it is legal

The king and the rook used for castling have not moved before.
No piece stands between the king and that rook.
The king is not currently in check.
The king does not cross or finish on an attacked square.

Standard (SAN) and Portable Game Notation (PGN), the plain-text format for exchanging games, use uppercase letters O: O-O. Tournament scoresheets governed by the International Chess Federation (FIDE) print 0-0 with zeroes. Both forms therefore circulate, but PGN requires O-O.

Common confusions

Uppercase O and zero

PGN requires uppercase O. FIDE scoresheets print zeroes, and some interfaces accept both forms, but not every program that reads PGN treats them as equivalent.

O-O and O-O-O

O-O is kingside castling and leaves the king on g1 or g8. O-O-O is queenside castling and leaves the king on c1 or c8.

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Sources

1.FIDE Laws of Chess taking effect from 1 January 2023, FIDE
2.Standard: Portable Game Notation Specification and Implementation Guide, rec.games.chess contributors, coordinated by Steven J. Edwards

Related terms

O-O-O