In the diagrammed position all possible moves of the black knights are set with mates: 1...Se-any 2.Rc3, 1..Sd-random 2.Rbb3 and 1...Sxb5 2.Qxe4. However, no mate is provided for 1...Kd5. Presumably the d8 knight will perform important guard duty in the extended black king field, so it is natural to turn to the h4 bishop as the likely key piece since it is very much out of play.
(A) 1.Bg3! Kd5 2.Qf7, (B) 1.Be7! Kd5 2.Rxc5 and (C) 1.Bf6 Kd5 2.Rxd4 are the three solutions.
Reason for choice: Note how in A the d-rook pins and the queen mates, in B the queen pins and the b-rook mates, and in C the b-rook pins and the d-rook mates. This interchange of function between the d-rook, queen and b-rook is the core of the problem. The key bishop works intensively; in each solution it crucially guards two squares in the mating net. A superb construction in which the b6 pawn is only a small blemish, necessary to prevent unwanted moves of the rook up the b-file after 1...Sd-random.
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