411 REFERENCE · Wikibooks
Chess Opening Theory/1. d4/1...d5/2. c4/2...e6/3. Nc3/3...Nf6/4. Nf3/4...Be7/5. Bf4/5...O-O/6. e3/6...c5/7. dxc5/7...Bxc5/8. Qc2/8...Nc6/9. a3
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Queen’s Gambit Declined, Blackburne Variation, Main Line
9. a3
This move is important to control the b4 square and support b2-b4. Black is forced to play 9...Qa5, which prevents b4 by pinning the a3 pawn to the rook and pins the c3 knight to the king. This also supports ...Nb4.