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

=Queen's Gambit Declined=

4. Bg5
With 4. Bg5 White pins Black's Knight against the Black Queen.

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