is, upset by one of the pieces involved being exchanged or sacrificed. An example of this is found in Diagram 6; KtxP
Diag. 7
fails on account of R X B; this leaves the Knight unprotected, and White wins two pieces for his Rook. Neither can the Bishop capture on K5 because of R X Kt. leaving the Bishop unprotected, after which BxKt does not retrieve the situation because the Rook recaptures from B6.
A second important case, in which our simple calculation is of no avail, occurs in a position where one of the defending pieces is forced away by a threat, the evasion of which is more important than the capture of the unit it defends. In Diagram 7, for instance, Black may not play KtxP, because White, by playing P- Q6, would force the Bishop to Kt4 or B1, to prevent the pawn from Queening and the Knight would be lost. A further example of the same type is given in Diagram 8. Here a peculiar mating threat, which occurs not
Diag. 8
infrequently in practical play, keeps the Black Queen tied to her KB2 and unavailable for the protection of the B at BI.
White wins as follows:
1. KtxB, KtxKt; 2. RxKt, QxR; 3. Kt-B7ch, K-Kt1; 4. Kt-R6 double ch, K-R1; 5 Q-Kt8ch, RxQ; 6. Kt-B7 mate.