The Knights are really more often under the necessity of making
several moves in succession than any other piece, because they
can never pass over more than one line at a time, and they may be
required to hasten from one wing of the board to the other just
as often as the other pieces. This is the reason why the most
favorable spot for a Knight is a square in the center of the
board; there he is always ready for an excursion to either wing.

The establishment of a Knight in the center can more readily be
effected in Queen's Pawn openings than in King's Pawn openings.
This will be evident from the following consideration:

In Queen's Pawn openings the squares e5 and e4 are the ones which
are aimed at by the respective Knights. If the opponent exchanges
the Knight with either his Queen's Knight or his King's Bishop,
the Pawn which takes the place of the Knight in recapturing,
gains control of two squares in the heart of the hostile camp. To
illustrate this by a case which often occurs: If after (1) P-d4,
P-d5; (2) Kt-f3, Kt-f6; (3) P-e3, P-c5; (4) B-d3, Kt-c6; (5) o-o,
P-e6; (6) P-b3, B-d6; (7) B-b2, o-o; (8) Ktb1-d2, P-b6; (9) Kt-e5
Black plays Bxe5, White in retaking drives Black's King's Knight
away depriving the King's wing of an important protection and
also creating a weakness on d6, where White might be able at some
later stage of the game to establish his Knight.

+---------------------------------------+
8 | #R | | #B | #Q | | #R | #K | |
|---------------------------------------|
7 | #P | | | | | #P | #P | #P |
|---------------------------------------|