Queenside castling

In standard chess, queenside castling is castling toward the queenside, where the rook on the a-file, the vertical column labeled a, begins. It counts as one move even though the king and rook both change squares.

Using standard square names, formed by a column letter and a row number, White moves the king from e1 to c1 and the rook from a1 to d1. Black moves the king from e8 to c8 and the rook from a8 to d8.

Conditions and the distinctive detail

• The king and the original queenside rook must still be on their starting squares and must never have moved earlier in the game.
• Every square between them must be empty: b1, c1, and d1 for White; b8, c8, and d8 for Black.
• The king cannot be in check or cross or finish on an attacked square: check e1, d1, and c1 for White, or the matching squares on Black's back row.

The b1 or b8 square must be empty because it lies between the rook and king, but it may be attacked because the king neither crosses nor occupies it. Queenside castling activates the rook on d1 or d8 and sends the king toward a different sector of the board, but it does not guarantee safety by itself.