The King is now on the Rook square of opposite colour to the Bishop, and must be driven to K R 8, or Q R square, in order to be checkmated.

12.Kt to B 7: ch.12.K to Kt sq.
13.B to B 5...

Purposely losing a move (coup de repos); it is immaterial where the Bishop moves to so long as it remains on the same diagonal, the object being to force Black to move, without altering White's position.

...13.K to B sq.
14.B to R 7...

To prevent the King from returning to Kt sq. if the Kt moves.

...14.K to K sq.
15.Kt to K 515.K to Q sq.
16.B to K 416.K to B 2
17.Kt to B 4...

The Black King is now gradually forced on to the fatal White corner.

...17.K to Q 2
18.K to B 718.K to Q sq.
19.B to B 619.K to B 2
20.B to Kt 5...

Not to R 4, because at Kt 5 the Bishop guards the additional square R 6.