chess move in which the king moves 2 squares toward a rook, while the rook moves to the square passed over by the king

Castling is a special move in chess that uses both a rook and the king. In castling, the king is moved two squares toward the rook, and the rook moves past the king to the square right next to the king.

White is castling on the queenside. The bottom image shows White's position after castling.
A chess castling move

Castling takes one move, and is the only way for a player to move two of his own pieces on the same move. Castling can be done on either side of the board.

Rules for castling change

Castling is only possible if each of the following things are true:

  • Neither the king nor the rook being used to castle have moved in the game.
  • The king is not in check, and is not moving into check or through check. (It is okay if the rook is being attacked, though).
  • There are no pieces between the king and the rook.

Benefits of castling change

Good players castle in most of their games. It is normally a good idea because it often moves the king into a safer place. The middle of the board is normally not a safe place for the king to be in the game. Castling also makes it easier to get the rook into the game. Castling king-side is generally viewed as being safer, and is more common than castling on the Q-side.

Castling was part of the changes made to the original version of chess. As the power of the pieces were increased, the kings had to be given a means of escape. Instead of being in the heat of battle, they needed to be tucked safely away. These changes were made in the 15th century (see history of chess).