The following middle game from a match of two masters furnishes
an example. After White's seventh move the position of Diagram 49
was reached, in which Black continued with P-b5 with the view to
playing B-g4 and Kt-d4. White replied (8) B-b3, B-g4; (9) Kt-e2.
Better would surely have been B-e3, which develops a new piece.
To allow the exchange of f3 which forces the g-Pawn out is
dangerous because Black may castle on the Queen's side and storm
with the Pawns of his King's wing.

+---------------------------------------+
8 | #R | | #B | #Q | #K | | | #R |
|---------------------------------------|
7 | | #P | #P | | | #P | #P | #P |
|---------------------------------------|
6 | #P | | #Kt| #P | | #Kt| | |
|---------------------------------------|
5 | | | #B | | #P | | | |
|---------------------------------------|
4 | ^B | | | | ^P | | | |
|---------------------------------------|
3 | | | ^Kt| ^P | | ^Kt| | |
|---------------------------------------|
2 | ^P | ^P | ^P | | | ^P | ^P | ^P |
|---------------------------------------|
1 | ^R | | ^B | ^Q | | ^R | ^K | |
+---------------------------------------+
a b c d e f g h

DIAGRAM 49.