White's second move attacks the e-pawn. Black usually defends this with 2...Nc6, which leads to several named openings. Of the alternatives, the most important are Petrov's Defense (2...Nf6) and Philidor's Defense (2...d6).
Of the alternatives, 2...Nf6 (Petrov's Defense) is considered fully respectable and is common in grandmaster games. Philidor's Defense is playable but is seen more rarely.
A number of less popular continuations are possible. These openings are generally considered to be less sound than those mentioned above. These openings are all categorized in the ECO under code C40.
