Here White has two bishops trained on the Black king’s position; he has a queen nearby, too. Look at captures you can make and their consequences. One of them is Bxg6. Black can’t ignore this, for now White is poised to mate with his queen on h7. So suppose Black recaptures f7xB, and now look for any checks that have become available. Qe6—checkmate.

A little lesson: in addition to being used in a sacrifice the queen also can fill the same role usually assigned to one of the bishops in these mates—and one of the bishops can be used to make the sacrifice more normally offered by the queen.

After 1. Bxg6 we have been assuming that Black would play the recapture f7xB. But he also has another, better reply that must be noticed: 1. …Nf6. If White now tries to renew his threat with BxN, it no longer works. Black plays f7xB and White no longer mates; when White plays his queen to e6, Black’s king can escape to h7 where his knight used to sit. (And if White tries Qh8, Black's king escapes to f7.) White’s best procedure in this variation is 1. Bxg6, Nf6; 2. Bh7+, Kg7; 3. Qg4+. This produces the position diagrammed in the next frame with Black about to move....