Chess Opening Theory/1. e4/1...e6/2. Nf3/2...d5

Black continues playing in the spirit of the French defence. White has a handful of options in this position, some better than others.

3. Nc3 enters the Two Knights variation where White continues developing pieces instead of pawns. Black has two responses: either kick White's knights around and grab a space advantage with 3...d5, or let White take over the centre after 3...Nf6 4. e5 Nfd7 in order to undermine it in a similar fashion to the advance variation.

3. d4 is the Perseus gambit. White gambits the e4-pawn and hopes to get compensation by putting their knight on e5 from where it can't yet be kicked by pawns (3...dxe4 4. Ne5 f6?? would lose to 5. Qh5+!).

3. e5 plays in a fashion similar to the advance variation, and may even transpose after 3...c5 4. d4, entering the Nimzowitsch system. However, White can deviate by playing 4. b4, the Wing gambit, instead.

1. e4 e6 2. Nf3 d5

Wikipedia has related information at French Defence.