The Latvian Counter-Gambit arises after 1.e4 e5 2.Nf3 f5. ECO code: C40.
Also known as the Latvian Gambit. Black immediately attacks the e4-pawn with ...f5, creating sharp, tactical complications. However, 2...f5 weakens the kingside severely. After 3.Nxe5 or 3.Bc4, White obtains excellent attacking chances. The gambit is considered objectively dubious but has devoted followers who enjoy its combative nature.