Queen's Indian Defense, Petrosian Variation

The Petrosian Variation begins by advancing White’s a2-pawn to a3. In algebraic notation, pawn moves use only the destination square, so 4.a3 means White plays a3 on the fourth move. The pawn controls b4 and prevents Black’s bishop from going there to pin a future knight on c3. This is a preventive move: it creates no immediate threat but removes a useful option from the opponent before White carries out a plan.

The logical continuation is Nc3, often followed by e4. White tries to build a mobile center, meaning a group of central pawns that can still advance and gain space. The benefit of a3 is that the c3-knight remains free. The cost is a tempo: White has moved a flank pawn while Black can develop with ...Bb7 or ...Ba6 and challenge the center with ...d5 or ...c5.

How Black responds

• ...Bb7 increases pressure on e4 and makes White prepare the central advance carefully.
• ...Ba6 attacks c4 and may draw White's queen or a pawn into defending it.
• ...d5 or ...c5 challenges the center before White finishes organizing it.

The move a3 is valuable only if White uses the freedom it creates. If Nc3 and e4 are delayed indefinitely, the invested tempo may have no justification. Compared with the Kasparov Variation, the Petrosian accepts slightly slower development to remove ...Bb4 from the start.