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| Kingside castling: O-O |
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| Queenside castling: O-O-O |
Castling is a special move in the game of chess involving the king and either rook. The move can be made only if all of the following conditions hold:
- The player must never have moved his king;
- The player must never have moved the rook with which he wishes to castle;
- There must be no pieces between the king and the rook;
- The king must not be in check, and castling must not put the king in check, or move the king over a square that is attacked by an enemy piece. Should the king be castling queenside (0-0-0) the squares a1 and b1 cannot be attacked either. Should the king be castling kingside (0-0) the square h1 cannot be attacked either. In short, the entire castling area must be "safe".
Under these conditions, castling involves moving the king two spaces toward the rook, then moving the rook into the square over which the king crossed. See the diagrams to the left.
To signal the intention to castle, one should pick up the king first and move it two squares, or pick up both the king and the rook simultaneously. Picking up the rook first signals the intention to just move the rook.
The notation for castling is O-O on the king side or O-O-O on the queen side.
Castling is an important goal in the early part of a game, because it serves two valuable purposes: it moves the king into a safer position away from the center of the board, and it moves the rook to a more active position in the center of the board.
If the king is forced to move before it has the opportunity to castle, the player may still wish to maneuver the king towards the edge of the board and the corresponding rook towards the center. When a player takes three or four moves to accomplish what castling would have accomplished in one move, it is sometimes called artificial castling.