castling

Castling is a special move in chess that involves the king and a rook moving as a pair to improve king safety and activate the rook. In the move, the king travels two squares toward the chosen rook, and the rook then moves to the square immediately next to the king on the opposite side.

Castling has specific conditions. It may be performed only if neither the king nor the chosen rook has moved previously in the game, there are no pieces between them, the king is not currently in check, and the king does not pass through or end on a square that is attacked. The squares between the king and rook must be unoccupied at the moment of castling, and the castling rights are lost if either the king or the corresponding rook has moved or if the rook has been captured.

There are two forms: kingside (short) castling and queenside (long) castling. In standard notation, they are written as O-O and O-O-O, respectively. For White, kingside castling moves the king from e1 to g1 and the rook from h1 to f1; queenside moves the king from e1 to c1 and the rook from a1 to d1. For Black, the same patterns apply to the squares e8, g8, f8, and the squares on the queenside.

Rights to castle are tracked separately for each side and can be lost if the king or the corresponding rook has moved, or if a rook is captured. Castling is a conventional means of safeguarding the king while mobilizing a rook, and its timing varies with strategic considerations.