Figure[Black to move]

Black is confronted with a fork by White’s bishop; he is about to lose the exchange to BxRf8. But instead of reaching to move the knight, think about Black's own offensive options. Again he has no checks, but ask anyway what else his pieces can threaten. In this case his dark-squared bishop can fork the White pieces also on dark squares: the knight and bishop. (Step back and scan the diagonals and other lines for pieces lined up on them.) The knight is loose, but of course the bishop is protected and anyway is an unsuitable target for a bishop fork. We handle this in the usual way: Black takes the bishop with RxB, to which White replies RxR. Now Be5 picks up the knight next move, netting two minor pieces for a rook.

Or perhaps things play out differently (the following sequence will take a bit of patience to follow): White can reply to 2. ...Be5 by leaving his rook on d6 and instead playing Nf5, taking his knight out of the fork and using it to give the rook protection. If Black replies e6xN, notice that White now has the rook fork Rd5—attacking the loose Black pieces that would then be on c5 (the knight) and e5 (the bishop). The pawn on e6 was guarding against this danger before it captured (in this variation) on f5. So instead of e6xN Black would want to respond to Nf5 with RxN. Now White has the recapture e4xR, but then Black has BxRd6, with the fork at last paying off. He ends up with two minor pieces against White's rook.

Now for extra credit, do you realize that the success of Black's fork here depends on his knight at c5 and his pawns on b7 and e6? Let's consider why. First, once White is forked he almost can move his rook both out of harm’s way and into position to protect his knight by playing Rd3. If this move were feasible the fork would be foiled; it is not feasible only because d3 is attacked by Black’s knight. This pattern nevertheless illustrates once again the extra difficulties created by double attacks against two ordinary pieces rather than the king: you have to ask what mischief either of them could make in breaking out of the fork.

And then notice, too, that the aforementioned Black knight on c5 is loose. This makes it a vulnerable target, so you need to worry that White might break out of the fork and make a counterattack against the knight. This almost is possible; once White’s rook is baited onto d6, it nearly can break out of Black's subsequent bishop fork with Rc6 or Rd5, threatening the knight. But not quite, because both of those squares are guarded by Black pawns. The lesson: we know that when you fork two enemy pieces (not the king), you need to ask whether one of those pieces might spoil the fork by rushing to the defense of the other; but consider as well whether one of the pieces in the fork might go off and make a threat against you elsewhere.