The moves 4.Nc3 Bb7 5.a3 (or 4.a3 Bb7 5.Nc3) are known as the 'Petrosian Variation'. Tigran Petrosian, World Champion from 1963 to 1969, had a unique style of play where he spent as much effort preventing his opponent's moves as he did furthering his own plans. The move a2-a3 in the Queen's Indian is a good example of his prophylactic style.
If Black continues 5...Be7, White plays 6.d5, blocking the fianchettoed Bishop and interfering with Black's Queenside development. Better is 5...d5. Now there are two important branches.
- 6.cxd5. Now 6...exd5 ends the fight for e4, but shuts in the Bishop. The move 6...Nxd5 is more fluid.
- After 7.Qc2 Nxc3 (better than 7...Nd7 8.Nxd5 exd5 9.Bg5 or 7...c5 8.dxc5 Bxc5 9.Bg5), whether White captures 8.bxc3 or 8.Qxc3, Black plays 8...Nd7, preparing ...c5 .
- After 7.e3 Black has the choice between 7...Be7 or a Bishop fianchetto with 7...g6.
- After 7.Bd2, Black should play 7...Nd7, not 7...c5, which allows 8.e4 Nxc3 9.Bxc3 Bxe4 10.Ne5, threatening a nasty check on the a4-e8 diagonal.
- 6.Bg5, and after 6...Be7, White can continue 7.Qa4+, 7.e3, or 7.Bxf6.
