Blackmar-Diemer Gambit Accepted

The Blackmar-Diemer begins 1.d4 d5 2.e4. After 2...dxe4 3.Nc3 Nf6, White plays 4.f3 and offers another pawn. The accepted variation is defined by 4...exf3. After 5.Nxf3, White has recaptured the black pawn that reached f3 but remains one pawn down in the overall material count.

In return, the g1-knight leaves its starting square and reaches f3, the f-file becomes semi-open for White's rook, and the bishops and queen can come out quickly. A semi-open file is a file containing no friendly pawn, although the opponent may still have one. White hopes to use this faster piece activity before Black securely protects the extra pawn and completes the setup.

Black does not need to cling passively to the material. The priorities are bringing pieces out, protecting the king, and exchanging attacking pieces when useful. Defenses with ...e6, ...Bf5, ...Bg4, or ...c6 create different setups but share the goal of reducing White's initiative, meaning the ability to create threats that force replies.

Acceptance refers specifically to the second capture on f3. If Black took on e4 but answers f3 without playing ...exf3, play enters the Blackmar-Diemer Gambit Declined. That distinction changes both the pawn count and the arrangement left on the board.