aka. Greco Counter-Gambit

Board Position
One of the rarer King's Pawn defences adopted by Black after an initial sequence of steps along the "Épine Dorsale" (a set of moves common to several openings). First given by Greco (1669), it was studied in the early 1900's by players from Riga. Board Position
Black answers the attack on his king's pawn with a KBP counter-attack against White's KP (move 2b, left), in a reply usually good for a draw with best play.
An early pawn exchange leads to open play (as shown typically by move 6w, right), while White often adopts increasingly artificial lines in order to avoid the drawing variations. Popular in correspondence chess because of its varied complications.

Clear or Groups
or see move: 1 e4 e5 2 Nf3 f5!? (to main:) 3 Nxe5 Qf6 4 d4 d6 5 Nc4 fxe4 6 Be2!
(to Keres:) 3 Bc4 fxe4 4 Nxe5 Qg5 5 d4 Qxg2 6.Qh5+ g6