Lesson
White to move. Main line: 1. Rh8#
Rh8 checks across the eighth rank. The black king cannot move to b8 because the rook controls that square, and it cannot step to a7 because the bishop on c5 controls the c5–b6–a7 diagonal.
The remaining flight square, b7, belongs to the white king on c6. This is why the king is an attacking piece in reduced positions: it can take away squares without making the mating move itself.
Before playing an edge-of-board rook check, trace the king’s two or three available flights. A rook supplies the wall, but another piece must close the diagonal escape and the attacking king often closes the last square.