A B C D E F G H

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

---------------------------------------
8 | | | #B | | #Q | #R | | #K |
|---------------------------------------|
7 | | | | |#Kt | | #P | #P |
|---------------------------------------|
6 | #P |^Kt | | | | | | |
|---------------------------------------|
5 | | | ^R | |^Kt | | | |
|---------------------------------------|
4 | | | ^Q | | | | | |