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Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...Nf6/4. Nf3/4...dxc4/5. e4/5...Bb4
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Vienna variation D37 Vienna variation
5...Bb4
Black pins the c3 knight, removing the only defender of the e4 pawn.
6. Bg5 is White's main response, pinning the f6 knight to the queen which indirectly defends the e4 pawn.
6. Bxc4 is a sideline, letting Black take the e4 pawn for faster development.
Theory table
6. Bxc4 6. Bg5
References
See also