1. That no piece of its own or the opponent's colour be between the King and the Rook with which the King is to be castled.
2. That neither the King nor the Rook with which it is to castle has yet moved.
3. The King cannot castle if in check, or into check; neither can he cross a square which is commanded by any of the opponent's pieces.
Command.—A piece is said to "command" a given square if it can take any adverse piece placed on such square.
Develop.—To move a piece from its original position is to develop it or bring it into play. The quicker the development of the pieces the better. If a player can concentrate upon a given point a greater number of pieces than his opponent is enabled to develop for its defence, he must obtain an advantage.
Double Pawn.—Two pawns on the same file. A "double pawn" is weaker than two pawns in the
same row, because in the former case they must be independently defended, whilst in the latter case either can be made to defend the other.
En prise.—A piece or pawn is en prise if so placed as to be liable to be taken by any other. Sometimes the same phrase is applied to a piece or pawn insufficiently defended.
To Exchange.—Is to give up a piece or pawn for another of equal value.
The Exchange.—A player gaining a Rook for a minor piece (Bishop or Knight) is said to "win the exchange." His opponent loses the exchange.