4.a3: the Petrosian Variation is variation 3 of 5 in the Queen's Indian Defence course, a advanced opening course for Black (ECO E12). It runs 26 moves and begins 1. d4 Nf6 2. c4 e6 3. Nf3 b6 4. a3 Bb7 …. White takes the b4 square from Black's bishop and plans a big centre. Black answers with ...d5, trades knights on c3 and strikes back with ...c5. White gets the centre and Black gets pressure against it. In the lesson every move is explained on a board and then drilled; it is free.
Move by move
1. d4 – White takes the centre.
1... Nf6 – Black develops and watches e4.
2. c4 – White gains space.
2... e6 – Black opens the f8 bishop.
3. Nf3 – White develops and avoids the pin on c3.
3... b6 – The Queen's Indian. The bishop will fight for e4.
4. a3 – The Petrosian Variation. White takes the b4 square away from Black's bishop.
4... Bb7 – Black keeps control of e4.
5. Nc3 – White develops and supports a push to e4.
5... d5 – Black fights for the centre at once and keeps White's pawn off e4.
6. cxd5 – White releases the tension.
6... Nxd5 – Black takes back with the knight and keeps the long diagonal open.
7. Qc2 – White prepares to push the e-pawn to e4.
7... Nxc3 – Black trades knights before White can play e4 and kick the knight.
8. bxc3 – White takes back toward the centre. The pawns on c3 and d4 support e4.
8... Be7 – Black develops and gets ready to castle.
The lesson goes on for 10 more moves, each with its explanation.
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