There is another common technique used to open the g-file and reach the same mating pattern: in the current position, White plays 1. Rxg7+. The rook draws protection from the bishop on the long diagonal, so the king can’t capture it and is forced into the corner. Now what? It might seem that White could mate just by withdrawing his rook, as this discovers check and seals off the g-file. But then Black has the reply mentioned a moment ago: f7-f6. How to prevent this interposition? There are two ways, both important to grasp.

(a) White can sacrifice the rook to bring the king back to g8: he plays 2. Rg8++, KxR; 3. Ra1-g1#. This sequence is known as Pillsbury’s mate for a game played by Harry Pillsbury that we will look at soon. Since the conclusive check is being delivered along the file rather than along the diagonal, interposing a pawn is no help to Black. This raises a general point to think about as you study our current mating idea. One of the two pieces you are using will be giving check while the other seals off the king’s only flight squares. If the piece that seals off the flight squares can be captured or if your opponent can interfere with its path, that isn’t a problem; he has no time to do those things because his king is in check by another piece. But if he can do those things to the piece trying to deliver the fatal check, it is a problem. If you are careful with your checks and sacrifices you may be able to avoid the difficulty by dictating the enemy king’s position and therefore controlling which of your pieces is playing which role, as seen here.

(b) There is another way to finish off Black after 1. Rxg7+, Kh8. White can take out the pawn on f7 with the discovered check Rxf7+; this forces Black to move his king back to g8. Now White returns his rook to g7, checking the king again and forcing it again to h8. This little sequence is a windmill, a theme discussed in the chapter on rook discoveries. It has enabled White to reset the position but without the pawn on f6. Now when White plays the discovered check Rg6+, Black still can interpose with Rf6; but this time the interposing piece has no protection, so White plays Bxf6 and mates. This sequence sometimes is known as Morphy’s concealed mate. If the names are getting confusing, don’t worry about them. Worry about the ideas.

These two ways of coping with a threatened interposition are worth close examination. The danger that your opponent will use a pawn to block the path of your bishop is a common complication in executing one of these mates using the long diagonal. The two methods just described come in handy; which is more helpful will depend on details of your position.