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. a3/9...Qa5
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
Queen’s Gambit Declined, Blackburne Variation, Main Line
9...Qa5
This move is forced, pinning the a3 pawn to the rook, preventing b2-b4 and supporting ...Nb4 ideas. White has a sideline and 2 mainline moves.
10. Nd2 is the sideline, avoiding the complications of the mainlines. As Nb3 is threatened, black responds with ...Be7.
The mainlines 10. Rd1 and 10. O-O-O are not for the faint of heart, as they lead to wild, extremely complicated positions.