Castling
Castling is the only move where two pieces go at once, and the only one with four separate conditions attached. Reading them is useless. Having a board refuse each one, and say which, is not.
Last updated
Castling is two pieces moving at once - the only move in chess that does that. Your king steps two squares towards a rook, and the rook hops over it to the square the king just crossed.
It counts as one move, and it counts as a king move: on a board you play
the king and the rook follows by itself. That is why you will see it written
O-O (kingside) and O-O-O (queenside) rather than as two moves.
Both are available in the position below. Try them.
Nothing is in the way and nothing is attacked. Castle - either side.
Notice how much further the king travels on the queenside: e1 to c1 leaves the rook on d1, right in the middle. That is the real difference between the two - kingside is tidier and faster, queenside puts a rook on a central file but leaves the king a little closer to the action.
The four conditions
| condition | who it is about |
|---|---|
| the king has not moved | the king |
| that rook has not moved | one rook - the other side is unaffected |
| every square between them is empty | both |
| the king is not in check, does not cross an attacked square, and does not land on one | the king only |
Read the last row again, because it is where almost every misunderstanding lives. Three separate things are forbidden to the king: being in check now, passing through an attacked square, and finishing on one.
And read what is not there: nothing at all about the rook being safe. The rook may be attacked, and it may cross an attacked square. Only the king is fussy.
Something in the way
The simplest refusal. Your own bishop is still sitting on f1.
This is why "castle early" in practice means "develop the two pieces on that side, then castle". The condition is not really a rule you fight - it is a to-do list.
Crossing an attacked square
The subtle one, and the reason a bishop on the long diagonal is so irritating. Black's bishop on a6 is looking straight down at f1. Your king is not in check. Nothing stands between king and rook. And you still cannot castle kingside.
Try castling kingside. Then castle on the side that is actually available.
The king is fine on g1. It is the journey that is illegal. That distinction does not exist anywhere else in chess, which is exactly why it catches people.
The other two refusals
| position | what the board says |
|---|---|
| your king is in check | you may not castle out of check. Answer the check first - move, block or capture |
| the king or that rook has moved earlier | the right is gone on that side, permanently. Putting the rook back does not restore it |
The second one is worth a warning: it is lost per side. A rook that shuffled
on the kingside has cost you O-O and nothing else - queenside is still on,
provided that rook and the king have both stayed home.
Try it
Black's bishop on b4 is checking your king down the diagonal. Both castling squares are empty and neither g1 nor f1 is attacked. Can you castle out of it?
Common questions
- How do you castle in chess?
- Move your king two squares towards a rook, then jump that rook to the square the king crossed. Kingside, the white king goes e1 to g1 and the rook h1 to f1. Queenside, the king goes e1 to c1 and the rook a1 to d1. On a board you play the king's move and the rook follows on its own - castling counts as one move, and it is always a king move.
- When can you not castle?
- Four cases. If the king or that particular rook has moved at any point earlier in the game. If any square between them is occupied. If your king is currently in check. And if the king would cross, or land on, a square an enemy piece attacks. Note what is missing from that list: the rook is allowed to be attacked, and it is allowed to cross an attacked square.
- Can you castle out of check?
- No. If you are in check you must answer the check first - move the king, block, or take the checking piece. Castling is not one of the legal answers, even when the king would land on a perfectly safe square.
- Can you castle if the rook has moved?
- No, and the right never comes back - not even if the rook returns to its original square. It is lost per side, so a rook that has moved on the kingside costs you kingside castling only; you can still castle queenside if that rook and the king are both untouched.
- Can you castle through check?
- No. The king may not pass through a square that an enemy piece attacks, which is what makes an enemy bishop pointing at f1 or f8 so annoying: nothing is attacking your king and you still cannot castle. The rook, by contrast, may cross an attacked square quite happily - the restriction is only ever about the king.
- Why is castling allowed at all?
- It is a speed-up, added in Europe over the 16th and 17th centuries and standardised in the 19th. A king left in the centre of the board is a target, and getting it to safety used to take three or four separate moves. Castling does it in one and develops a rook at the same time, which is why nearly every serious game features it.
