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 .

Kingside castling

O-O on the kingside

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

Start

O-O

f1 and g1 are empty, and the king can cross them safely.

Initial arrangement

The white king is on e1, the rook is on h1, f1 and g1 are empty, and all three king squares involved are safe.

1. f1 and g1 are empty, and the king can cross them safely.

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.

Queenside castling

O-O-O on the queenside

8

7

6

5

4

3

2

a1

b

c

d

e

f

g

h

Start

O-O-O

b1, c1, and d1 are empty; the king travels e1-d1-c1.

Initial arrangement

The white king is on e1, the rook is on a1, b1, c1, and d1 are empty, and e1, d1, and c1 are not attacked.

1. b1, c1, and d1 are empty; the king travels e1-d1-c1.

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.