Figure[White to move]

Now the same idea in a setting with some more complexity. In the previous frame the open board and the alignment of White’s pieces on the e-file plainly suggested a combination against Black’s king; here the makings of mate are a little less obvious but the mechanics are similar. White sees that Black’s back rank is a bit weak. There is a rook on f8, but again no other Black piece is in a position to defend d8—and White has a rook aimed there. White thus starts by eliminating Black’s rook and smoking out Black’s king with 1. QxR+, requiring KxQ. Now White pushes the king back under its pawn cover with 2. Bc5+. Notice that this time the other bishop—on b5—does crucial work that was performed in the previous position by the rook on e1: it prevents Black’s king from escaping onto e8. If Black plays Kg8, White mates with 3. Rd8#.

Why “if” Black plays Kg8? Because this time Black has another option: in reply to Bc5+ he can interpose his queen at d6. You might think White could respond by simply renewing the check with BxQ+, and he can. Notice the power of the move: it wins back the queen, leaving White up a rook; more to the point, it’s a pretty bishop fork of Black’s other rook and his king on its new square. So White sees that he is going to win two rooks and is ready to go forward. For all that, though, BxQ+ isn't White’s best move, for once his bishop has captured on d6 it blocks the rook’s path to d8. Now after the Black king retreats to g8 (this time it’s forced), White has no mate. He can move his bishop out of the way by taking the rook on b8; but since that isn’t a check it gives Black time to play Nd7, again blocking the rook’s path and doing it with protection. Nor can White then take the knight with his light-squared bishop. The bishop became pinned when Black moved his knight.

So the even more decisive reply to Black’s Qd6 is 3. RxQ, pictured in the next frame....