Now, Black has to choose between the two pawns which to capture : 3...exd4 or 3...Nxe4. Unless he wants to keep on playing a symmetrical game with 3...d5.
A transposition to Philidor's Defence is possible by the mean of 3...d6
For explanation of theory tables see theory table and for notation see algebraic notation.
1. e4 e5 2. Nf3 Nf6 3. d4
When contributing to this Wikibook, please follow the Conventions for organization.
Wikipedia has related information at Petrov's Defence