Black captures the pawn and material is now equal. White would like to take some control of the centre, develop, and ideally force Black to move or lose their advanced knight. They have several options.

5. d4, the Classical attack, is the most principled move and the mainline. White prioritises taking space in the centre. Next they may develop their bishop behind their pawn. After 5...d5 6. Bd3 White is ready to castle, but Black's knight is dug in on e4. White will try to increase pressure on the knight with moves like c4 and Nc3, hoping to win some concession.

5. Nc3, the Nimzowitsch attack, is the chief sideline. White offers a trade of knights, and after 5...Nxc3 6. dxc3, Black's knight is gone and White has doubled pawns in exchange for rapid development.

Alternatively White can prioritise kicking the knight first.

5. Qe2, the Cozio attack, is the most common continuation at the amateur level. White pins Black's knight to the queen. Defending it with 5...d5? fails to 6. d3. Black's only move to unpin and defend the knight is 5...Qe7. After 6. d3 Nf6, the position leads to an exchange of queens and an early endgame.

5. d3, the French attack, kicks the knight away immediately with a pawn. After 5...Nf6, the main moves are 6. d4, important for opening up White's bishop, and 6...d5, transposing into an Exchange French.

5. Bd3, the Millennium attack is a rarely played sideline. Compared to 5. d4 and 6. Bd3, Black isn't in time to play 5...d5 to defend the knight, so has to move it away instead. However, as White's bishop now obstructs their d-pawn they will soon have to move it again (usually to c3: 5...Nf6 6. O-O Be7 7. c3 O-O 8. Bc2 Bg4 9. d4).

5. c4, the Kaufman attack, the idea being to prevent Black from advancing his pawn to d5 so that so that after d4 and Bd3, Black's knight is still loose and they must move it.

For explanation of theory tables, see theory table and for notation, see algebraic notation.

1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4

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Wikipedia has related information at Petrov's Defence

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.

• Yusupov, Artur (1999). The Petroff Defence. Edition Olms. ISBN 3-283-00400-5.