What is Castling Long?

Castling long (or queenside castling) is the form of castling in which the king moves two squares toward the a-file rook. Notated as O-O-O. The king lands on the c-file and the rook on the d-file. Queenside castling is less common than kingside castling because the king is less sheltered (the a-pawn is farther away) and more preparation is required (three squares between king and rook must be clear).