ECO C43 · Petrov's Defense
Petrov's Defense: Modern Attack, Center Variation
The position after 1.e4 e5 2.Nf3 Nf6 3.d4 Nxe4 4.Bd3.
1. e4 e5 2. Nf3 Nf6 3. d4 Nxe4 4. Bd3
Petrov's Defense: Modern Attack, Center Variation is filed under ECO C43, a variation of Petrov's Defense: Modern Attack. It is the position after 1.e4 e5 2.Nf3 Nf6 3.d4 Nxe4 4.Bd3 — 7 half-moves in, with Black to move. From here theory names one continuation, Nc6.