Figure[White to move]

Exchanging Away the Guard.

Now let’s assume an enemy piece guards the square your knight needs, and it isn't pinned. Perhaps you nevertheless can get rid of it. Sometimes the guardian of the forking square may be captured: you can take it, and the piece that recaptures yours no longer will protect against the fork.

Start with the diagram to the left. The position of Black’s king and rook make the idea for White clear enough: Nf7+. But f7 is protected by Black’s knight. Ask if it can be captured, and see that it can be―with White’s rook. After playing RxN, White loses the rook to f6xR; but he regains it with the fork Nf7+, capturing Black’s rook next move and leaving White a knight to the good.

Remember when you play a capture that your opponent may not be required to recapture. Usually that will be his choice, but in principle he also may be able to make some other capture or counterthreat of his own. Here Black can reply to White’s RxN by playing RxN himself. Doesn't this end the forking threat? It does, but at a prohibitive price; for then White has Re8#—a classic back rank mate that takes advantage of the way Black's king is stuck in the corner. At the outset of the position the Black rook on d8 is the only piece protecting against this mating threat, so it can't afford to leave its post. We will study back rank mates in detail at various points later in this project (they get a section to themselves toward the end).