For playing a man to a square to which it cannot be legally moved, the adversary, at his option, may require him to move the man legally, or to move the King.

For touching an adversary's man, when it cannot be captured, the offender must move his King.

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.

A player is said to have the opposition when he can place his King directly in front of the adverse King, with only one square between them. This is often an important advantage in ending games.