In the King's Gambit (after 1.e4 e5 2.f4) there are many ways to decline the free pawn. Some are:
The Classical variation: 2...Bc5
Norwalde Variation 2...Qf6
Panteldakis Counter Gambit 2...f5
Nimzowitsch Counter Gambit 2...d5 3 exd5 c6
Adelaide Counter Gambit 2...Nc6 3.Nf3 f5
Keene Defense 2...Qh4+ 3.g3 Qe7
Mafia Defense 2...c5
Petrov Defense 2...Nf6
...each giving White certain advantages and certain problems.
The Petrov Defense has one of the worst records of them all, almost to the point where one might conclude that it's been refuted. However it does have some intriguing points.
The Petrov Defense in the KGD is defined already by Black's second move, 2...Nf6:
Greco gave a fragment of this opening in 1620:
Nimzowitsch felt the opening was immediately losing for Black. In "My System" he gives the following example:
In his book "How to Beat Bobby Fischer," Edgar Medis call the move 2...Nf6 dubious. Still some players tend to give the Petrov Defense a try.
After 2...Nf6, White has a couple of proven options: 3.fxe5 and 3.Nf3.
Moves that transpose:
The Petrov Defense bears a strong resemblance to the Vienna Gambit, the main difference being that White has played Nc3 (and reached the same position transposed as 1.e4 e5 2.f4 Nf6 3.Nc3) :
White has the option to play 3.d3 which leads to a position transposed from the Maroczy Variation of Alekhine's Defense. Below is the original game between Maroczy and Alekhine: