White has brought his minor piece into action and taken control of the middle. Simultaneously, he has put pressure on Black's e4-pawn, which Black must now try and defend. Nf6 or Bf5 would most commonly be used to defend the lone pawn, while e5 gives the option of a pawn transfer. f5 defends the pawn using Black's f-pawn, a less optimal response. Rarer responses include Nc6 transposing to the Nimzowitsch Defense and adding an attacker to the d4 pawn, and c5, a rather unexplored countergambit.

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1. d4 d5 2. e4 dxe4 3. Nc3

Wikipedia has related information at Blackmar-Diemer Gambit.

• Kasparov, Garry, & Keene, Raymond 1989 Batsford chess openings 2. ISBN 0-8050-3409-9.