411 REFERENCE · Wikibooks
Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...Nf6/4. Bg5
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Queen's Gambit Declined D50-D69 Queen's Gambit Declined
4. Bg5
With 4. Bg5 White pins Black's knight against the black queen.
Theory table
4...Be7 4...Nbd7
References