Diemer-Duhm Gambit Accepted

After 1. e4 e6 2. d4 d5, the Diemer-Duhm Gambit 3. c4 leaves the e4-pawn undefended and instead attacks d5 from the flank. With 3...dxe4, Black accepts the pawn. Unlike the main third moves, which defend, advance or exchange the e-pawn, 3. c4 gives up a central pawn in exchange for development and the c4/d4 pawn pair. Wikibooks marks the move as dubious. The engine gives Black a slight edge after the capture. On Lichess, however, the results are almost evenly split, which suggests that in practice White's activity makes up for the pawn.

The position has half-open d- and e-files. Black's doubled e-pawns leave an advanced pawn on e4, and that pawn is White's natural target. By far the most common reply is 4. Nc3, which develops and attacks e4 with the knight. 4. f3 hits the same pawn with another pawn and offers to open the f-file. 4. d5 gains space and strikes at e6 instead of recovering material, and it has given White the best practical results of the three. The line appears mostly in club and online play, not at a serious level.