Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...Nf6/4. Bg5/4...Nbd7

=Queen's Gambit Declined=

4...Nbd7
With 4...Nbd7, black intends to play the Cambridge Springs Defense, which continues 5. Nf3 c6 6. e3 Qa5. The Cambridge Springs Defense is a solid line of the QGD and is often played both in the amateur level (because of the easier to understand theme) and the grandmaster level. This move also sets a trap that has ensnared quite a few players. White can continue 5. cxd5 exd5, resulting in a position that resembles the QGD Exchange variation (which is fine for both camps), as long as he/she doesn't play 6. Nxd5??. While it superficially appears to win a pawn (because the black knight is pinned), black can capture with his knight with 6... Nxd5!. Then after 7. Bxd8, white has not won black's queen because of 7... Bb4+!, where the only legal move is 8. Qd2, and black gets his/her queen back and emerges a minor piece ahead.

Theory table
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'''1. d4 d5 2. c4 e6 3. Nc3 Nf6 4. Bg5 Nbd7'''