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Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...Nf6/4. Nf3/4...Be7/5. Bf4
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Queen's Gambit Declined D37 Queen's Gambit Declined Blackburne Variation
5. Bf4 · Harrwitz Attack
White develops the bishop to f4. This just looks like a standard developing move, but it is extremely aggressive. With the bishop on f4 instead of g5, White avoids piece exchanges, and often castles queenside while Black castles kingside, leading to sharp opposite-side castling positions.
Black has nothing better to do than castle.
Theory table
5...O-O
References
See also