19. … Castles? 20. QxPch PxQ 21. B-R6 Mate
GAME No. 21
White: Eduard Lasker. Black: Aljechin.
Three Knights’ Defence.
1. P-K4 P-K4 2. Kt-KB3 Kt-QB3 3. Kt-B3 B-Kt5 4. Kt-Q5
Developing another piece by B-Kt5 or B4 would be more in accordance with principle.
4. … B-K2
There was a threat of KtxB and KtxP. If Black plays P-Q3, the B must retire all the same after 5. B-Kt5. It seems best to retire the B to K2 rather than to B4 or R4, because there remains the threat of a pin subsequently by B-KKt5, which might become serious with the Knight at Q5.
5. B-B4 Kt-B3 6. P-Q3 P-Q3 7. KtxB QxKt 8. P-B3 P-KR3
The KKt is to support the advance of P-Q4 subsequently, and that is why Black does not want to allow it to be pinned. This is sound strategy, since White has exchanged his QKt, which from B3 prevents P-Q4 in the ordinary way.