Petrov's Defense

Named after the 19th-century Russian master Alexander Petrov; also called the Russian Game. Black answers the attack on e5 with a counterattack on e4 and is known for its solidity.

It is reached after 4 moves: 1. e4 e5 2. Nf3 Nf6.

White is to move and has 27 legal moves. Over the next four moves — two for each side — the game can branch into 603,843 different games. Those games reach 214,159 different positions: most positions can be reached in more than one way, 2.8 on average.

The Lichess opening list names 51 further lines after this position, such as Modern Attack (3. d4) and Italian Variation (3. Bc4).

The same moves can be played in 4 different legal orders that all reach this position. For example 1. Nf3 e5 2. e4 Nf6, which starts as the Zukertort Opening.