Castling rules
Four conditions. Two of them are the ones students forget.
Castling moves the king two squares toward a rook, and the rook hops over to the other side. Short castling (O-O) goes toward the h-file rook; long castling (O-O-O) goes toward the a-file rook.
You cannot castle if…
- The king has already moved — even if it moved and came back
- That rook has already moved — the other rook may still be available
- Any square between the king and rook is occupied
- The king is in check, passes through an attacked square, or lands on one
What is allowed, and surprises people
- The rook may be attacked — that is fine
- For long castling, the b1 or b8 square may be attacked — the king does not cross it
- The rook may pass through an attacked square; only the king may not
Worked examples
Which move below is possible for White?
- a) Short Castle (O-O)
- b) Long Castle (O-O-O) ← correct
- c) Take Black's Pawn
- d) Take Black's Bishop
Long castling is legal: the squares between King and Rook are empty, the King is not in check, and it does not cross or land on an attacked square. O-O is illegal because a piece is standing between the King and that Rook. Neither listed capture is possible: no White piece can reach those Black pieces this move.
Which move below is possible for White?
- a) Short Castle (O-O)
- b) Long Castle (O-O-O) ← correct
- c) Take Black's Pawn
- d) Take Black's Rook
Long castling is legal: the squares between King and Rook are empty, the King is not in check, and it does not cross or land on an attacked square. O-O is illegal because a piece is standing between the King and that Rook. Neither listed capture is possible: no White piece can reach those Black pieces this move.
Which move below is possible for White?
- a) Short Castle (O-O) ← correct
- b) Long Castle (O-O-O)
- c) Take Black's Knight
- d) Take Black's Rook
Short castling is legal: the squares between King and Rook are empty, the King is not in check, and it does not cross or land on an attacked square. O-O-O is illegal because a piece is standing between the King and that Rook. Neither listed capture is possible: no White piece can reach those Black pieces this move.
Common questions
When can you not castle in chess?
If the king or that rook has already moved, if a piece is between them, or if the king is in check, would pass through an attacked square, or would land on one.
Can you castle if the rook is attacked?
Yes. The restriction on attacked squares applies only to the king's starting, passing and landing squares — not to the rook.
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