R-Q1, because White could bring another two pieces to bear on the
P, the other Rook and the Knight, whilst Black has only one more
piece available for the defence, namely, his Rook.

The following examples show typical positions, in which simple
calculation is complicated by side issues.

In Diagram 10, the point of attack, namely, the Black Knight at
KB3, can be supported by as many Black units as White can bring
up for the attack, but the defensive efficiency of one of Black's
pieces is illusory, because it can be taken by a White piece. The
plan would be as follows: White threatens Black's Knight for the
third time with Kt-K4, and Black must reply QKt-Q2, because
covering with R-K3 would cost the "exchange," as will appear from
a comparison of the value of the pieces concerned. The "exchange"
is, however, lost for Black on the next move, because

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