Queen's Indian Defense:Petrosian Variation

Named after Tigran Petrosian: 4. a3 prevents ...Bb4. Garry Kasparov played it often early in his career.

It is reached after 7 moves: 1. d4 Nf6 2. c4 e6 3. Nf3 b6 4. a3. It is a line of the Queen's Indian Defense.

Black is to move and has 29 legal moves. Over the next four moves — two for each side — the game can branch into 815,892 different games. Those games reach 270,031 different positions: most positions can be reached in more than one way, 3 on average.

The Lichess opening list names 13 further lines after this position, such as Kasparov-Petrosian Variation (4... Bb7 5. Nc3) and Farago Defense (4... Ba6 5. Qc2 Bb7).

The same moves can be played in 144 different legal orders that all reach this position. For example 1. c4 Nf6 2. d4 e6 3. Nf3 b6 4. a3, which starts as the English Opening.