|---------------------------------------| 4 | | ^P | | | | | | | |---------------------------------------| 3 | ^P |^Kt | | | | ^P | ^B | | |---------------------------------------| 2 | | | ^R | | | | ^P | ^P | |---------------------------------------| 1 | | | ^R | | | | ^K | | --------------------------------------- A B C D E F G H

Diag. 9.

It would be obviously wrong to move the pawn to Q6 after White’s 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|    |
|---------------------------------------|
2 | ^P | ^P | ^P |    |    | ^P | ^P | ^P |
|---------------------------------------|
1 | ^R |    |    | ^Q |    | ^R | ^K |    |
---------------------------------------
A    B    C    D    E    F    G    H

Diag. 10

White’s further attack on the Knight by Q-B3 forces the Rook to defend on K3, where it gets into the diagonal of the Bishop, which at present is masked by White’s Knight.  The sequel would be 3.  QKtxKtch, RxKt (not BxKt on account of BxR winning a whole Rook), 4.  BxR, and so on.  A similar case is shown in Diagram 11.

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