Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nf6/3. Nxe5/3...d6/4. Nf3/4...Nxe4/5. Nc3

5. Nc3
This move continues the main line of the Petrov's defence. Black can trade knights to double White's pawn or retreat their knight to f6.