A very powerful move, which brings us to the middle-game stage, as there is already in view a combination to win quickly by Kt - Q 5. This threat makes it impossible for Black to continue the same course. (There is a long analysis showing that Black should lose if he also plays B - Kt 5.) He is now forced to play 7...B × Kt, as experience has shown, thus bringing up to notice three things.
First, the complete development of the opening has taken only seven moves. (This varies up to ten or twelve moves in some very exceptional cases. As a rule, eight should be enough.) Second, Black has
been compelled to exchange a Bishop for a Knight, but as a compensation he has isolated White's Q R P and doubled a Pawn. (This, at such an early stage of the game, is rather an advantage for White, as the Pawn is doubled towards the centre of the board.) Third, White by the exchange brings up a Pawn to control the square Q 4, puts Black on the defensive, as experience will show, and thus keeps the initiative, an unquestionable advantage.[[1]]
The strategical principles expounded above are the same for all the openings, only their tactical application varies according to the circumstances.
Before proceeding further I wish to lay stress on the following point which the student should bear in mind.
Before development has been completed no piece should be moved more than once, unless it is essential in order to obtain either material advantage or to secure freedom of action.
The beginner would do well to remember this, as well as what has already been stated: viz., bring out the Knights before bringing out the Bishops.
7. CONTROL OF THE CENTRE