The King can be Castled only:--

When neither the King nor the Castling Rook has been moved, and

Where the King is not in check, and

Where all the squares between the King and Rook are unoccupied, and

Where no hostile man attacks the square on which the King is to be
placed, or the square he crosses.

In Castling, the King must be moved first, or before the Rook is
quitted. If the Rook be quitted before the King is touched, the
opposing player may demand that the move of the Rook shall stand
without the Castling being completed.

The penalty of moving the King prohibits Castling.


EN PASSANT.

Taking the Pawn "_en Passant_," when the only possible move, is
compulsory.


PROMOTING THE PAWN.

A Pawn reaching the eighth square must be at once exchanged for any
piece of its own color (except the King) that the player of the
Pawn may elect.