411 REFERENCE · Wikibooks
Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bb5/3...Nd4/4. Nxd4
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Bird's defence Bird's defence C61
4. Nxd4
White takes the offered trade. This is the main continuation.
After 4...exd4, Black will have gained some space and the pawn on d4, which is not so easy for White to dislodge.
Theory table
4... exd4 5. O-O Bc5 6. d3 c6 7. Ba4 Ne7 8. f4 f5 9. Bb3 d5 Bird's defence + 4. ... ... 5. ... ... 6. ... ... 7. Bc4 d5 8. exd5 cxd5 9. Bb5+ Bd7 4. ... ... 5. ... c6 6. Bc4 Nf6 7. Re1 d6 + 4. ... ... 5. ... a5 6. d3?? c6 7. Bc4 b5 -- 4. ... ... 5. ... Ne7 Paulsen variation
References
Ruy Lopez