Castling

Castling is one move that relocates the king two squares toward a rook and places that rook on the square the king crossed.

The king and chosen rook must not have moved, the squares between them must be empty, and the king may not start in check, cross an attacked square, or finish in check.

Castle on either side

In castling, the king moves two squares toward a rook and that rook moves to the square the king crossed. Kingside castling is O-O, while queenside castling is O-O-O in PGN.

Move the white king to g1 or c1 to castle.

White may castle to g1 or c1 in this simplified legal position.

All castling conditions must be satisfied

The king and chosen rook must not have moved earlier, and every square between them must be empty. The king may not be in check, cross an attacked square, or finish on an attacked square.

The rook may be attacked, and on queenside castling the b-file square may be attacked, because the king does not occupy or cross that square.

Castling rights do not return

Moving the king removes both castling rights for the rest of the game. Moving a rook removes the right to castle with that rook, even if the piece later returns to its starting square.