In chess, castling is a special move that involves the king and the rook. It is the only move in chess where two pieces can move at once. To castle, the king is moved two spaces towards the rook, and the rook is then moved to the other side of the king. Castling can be done on either side of the board and with either rook, but there are rules that must be followed:

• The king and the rook must not have moved before.
• There can be no pieces between the king and the rook.
• The king cannot be in check before or after castling.
• The king cannot pass through a square that is attacked by an opponents piece.

If any of these rules are violated, castling is not allowed. Castling is an important move in chess, as it can help to protect the king and bring the rook into play.