Castling is a special move in Chess that involves two pieces at once: the king and one of the rooks. The move exchanges the order of the two pieces on the back rank as if they jumped over each other.
Castling is performed in the following way: the king moves two squares towards the rook, and the rook jumps over the king and moves to the adjacent field (to the field which the king has crossed). It has two types depending on the rook:
White castling short
White castling long
Black castling short
Black castling long
Castling is allowed only if the following conditions are met:
White cannot castle, because the king is in check.
White cannot castle, because the king would move through check.
White can only castle to kingside, because on queenside, the king would move into check.
White can castle to queenside, because the king does not move throgh check. (The path of the rook does not matter.)
Castling is a very strong move as it – uniquely in chess – makes two pieces to move. It moves the king into a safe place, and activates the rook at once.
Though the castling rules can be very different in chess variants, some general rules can be given (but there are exceptions):
Some examples of different, unusual castling rules: