Castling in chess: what it is, when you can and how to do it

1. What castling is

To castle, the king moves two squares towards one of its rooks, and that rook jumps over to the other side of the king, right next to it.

Kingside (short) castling goes towards the nearer rook: the white king ends on g1 and the rook on f1. Queenside (long) castling goes towards the queen’s rook: the white king ends on c1 and the rook on d1. Black does the same on rank 8.

2. When you can castle

You may castle only if all of these are true:

• The king has not moved at all in the game.
• The rook you castle with has not moved.
• There are no pieces between the king and that rook.
• The king is not in check right now.
• The king does not pass through or land on an attacked square.

3. Common mistakes

The rook may be attacked, and in queenside castling the b1 (or b8) square may be attacked: what matters are the squares the king crosses.

If you move the king "just to try" and move it back, you lose the right to castle for good. The same goes for each rook on its own: if only the queen’s rook has moved, you can still castle kingside.

4. When to castle

In most games it pays to castle within the first ten moves, after developing the knight and bishop on that side. The king sits behind its pawns and the rook reaches the center.

Kingside castling is quicker (fewer pieces to move out) and keeps the king safer. Queenside castling is more aggressive: the rook reaches the d-file at once, but the king is a little more exposed.

5. How to castle on Vexply

Tap or drag the king two squares towards the rook; the game moves the rook for you. If castling isn’t allowed, those squares aren’t highlighted. In the move list, kingside castling is written O-O and queenside O-O-O.