Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...Nf6

= Queen's Gambit Declined =

3...Nf6
With 3...Nf6 Black begins the development of his minor pieces, continues to fight for the center, including the battleground squares d5 and e4, and gets closer to castling.

This is the classical main-line of the QGD. White has several options, though some of these may transpose into each other at some point:
 * 4. cxd5 - The exchange variation. Unlike other exchange variations notorious for yielding a quiet game (e.g. French, Slav), this exchange has several aggressive plans including the "minority attack" on the queenside or a preparation for an all-out assault by seizing the center by an eventual e2-e4 or e3-e4.
 * 4. Nf3 - This is the solid classical option.
 * 4. Bg5 - Since this move is usually played anyway, it is both constructive while keeping options open.

Theory table
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'1. d4 d5 2. c4 e6 3. Nc3 Nf6'