Opening
The first stage of a game, during which the players try to develop their pieces and lay the groundwork for later operations (seizing the center, building a favorable pawn structure). The opening obeys the general laws of chess strategy and tactics, and above all the key strategic principle of piece coordination with the pawns.
Seizing the center with pawns
The fight for the center matters because holding it firmly with pawns wins a great deal of space. That makes it easier to maneuver and coordinate your forces, and harder for the opponent to do the same.
The fight for the center can be direct, with both sides trying to seize it right away.
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The game takes on a different character when one side tries to advance the central pawns while the other hinders it. In such cases the squares next to the center — the extended center — take on real importance.
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There are openings in which one side seizes the center while the other then tries to break it up (undermine it) — the Alekhine Defense, for example.
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Finally, both White and Black sometimes hold off on seizing the center, waiting to do it under the most favorable circumstances.
Centralization of the pieces
Placing pieces (especially the knight) in the center of the board is highly important, provided the opponent cannot drive them back. A piece in the center usually commands more squares and can be transferred more easily to any sector of the board. Building the most favorable pawn structure often coincides with the opening's first task — the fight for the center. What matters here is the soundness of the pawn formation, whether the pawns can advance further to put pressure on the opponent's position, and the coordination of pawns and pieces.
Duration of the opening
There is no precise boundary separating the opening from the middlegame. Sometimes pressure on the opponent's position begins before development is fully complete. At other times the opening's tasks are all done, yet neither player hurries into action, and it is hard to say whether the opening is still going on or the middlegame has already begun.
In general these two stages are closely linked, since an experienced player plays the opening with a particular kind of later play in mind. Moreover, an opening is sometimes played with an eye to advantages and drawbacks that will only really matter in the endgame.
So the duration of the opening can only be given approximately — 10 to 15 moves.