Petrov's Defense: Classical Attack, Mason Variation

The Petrov's Defense: Classical Attack, Mason Variation is a chess opening classified under ECO code C42 in the Open Games & French Defense family, beginning with 1.e4 e5 or 1.e4 e6, including the Italian Game, Ruy Lopez, and French Defense. It is reached after 7 moves: 1. e4 e5 2. Nf3 Nf6 3. Nxe5 d6 4. Nf3 Nxe4 5. d4 d5 6. Bd3 Be7 7. O-O O-O. The resulting position leaves white to move.