Chess rules
Castling in Chess: Rules, Kingside & Queenside
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Castling is the only move where you move two pieces at once: the king steps two squares towards a rook, and that rook moves to the square on the king’s other side. It often improves king safety and rook activity, but several conditions must be met and castling is not automatically the safest choice.
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Kingside and queenside
There are two ways to castle. Kingside (“short”, written O-O) is towards the h-file rook: for White, the king goes to g1 and the rook to f1. Queenside (“long”, written O-O-O) is towards the a-file rook: for White, the king goes to c1 and the rook to d1. Queenside requires three intervening squares to be empty rather than two, and the two destinations leave different pawn shelters and rook placements.
The five rules that forbid castling
Castling is legal only if every one of these is satisfied. The first two cost you the right permanently; the last three only block castling for the moment.
- The king has not moved. Once your king moves, you can never castle again — on either side.
- That rook has not moved. Moving a rook forfeits castling on that side only.
- No pieces between the king and the rook — the squares must be empty.
- The king is not in check. You cannot castle to escape a check.
- The king does not pass through or land on an attacked square. No castling through check or into check.
What is and isn’t allowed
A few clarifications that trip people up: the rook is allowed to be attacked, and for queenside castling the rook passes over b1 or b8 — only the king’s starting, crossing, and destination squares are tested for attack. A king that was in check earlier in the game may still castle later if it did not move and all the conditions now hold. Castling is often useful for king safety, but the position decides whether and where to castle.