Mate in 4

1.Bd5+ Bxd5 2.Nxd7, with threats of 3.Nc5# and 3.Nf6#, is refuted by 2…Rh7!, pinning the wN. Likewise, 1.Bd5+ Bxd5 2.Na4, aiming for 3.Nc3# and 3.Nc5#, is met with 2…Rc1!, covering both c3 and c5. Thus, h1 is the ideal perch for Black’s rook, from which it keeps watch over both h7 and c1. Draw it away from this vantage point, and Bd5+ followed by either Nxd7 or Na4 will succeed.

This is precisely what the key move 1.Bf7!, threatening 2.Bxg6#, achieves. Any defence now compels the h1 rook to abandon its post, leading to:

• 1...Rg1 2.Bd5+! Bxd5 3.Nxd7!, after which either 3.Nc5# or 3.Nf6# follows; Black cannot avert both.

• 1...Rh6 2.Bd5+! Bxd5 3.Na4!, with 3.Nc3# or 3.Nc5# in prospect; Black may parry one, but never both.

Needless to say, 1…Rh5 fares even worse, allowing a swifter mate: 2.Bxg6+ Rf5 3.Bxf5#.

A crisp more-mover, and a textbook specimen of the focal theme: a line-piece at a focal position simultaneously guards two or more squares (foci) along different lines, and the opponent’s plan is to lure it from that station, leaving at least one focus unguarded.