Also simply called castling; it is a special move involving the King and one Rook of choice. It is the only move allowing the King to move 2 squares at a time. There are certain limitations when it is not permitted:
- the King or the Rook in question have previously moved (even if they return to their original squares)
- the King is in check, or will pass through check
- there is a piece of either color between the King and Rook