Blackmar-Diemer Gambit Declined

The Blackmar-Diemer Gambit Declined covers the replies to 4. f3 in which Black does not capture on f3. These moves leave the e4-pawn where it is, so White can recapture it with fxe4 or Nxe4, or keep developing and offer the exchange later. The most frequent choice is 4...Bf5, the Vienna Defense, which Matthias Wahls recommended in his writing. Next come 4...e6, the Weinsbach Defense, and 4...c6, the O'Kelly Defense.

The O'Kelly is often presented as a refined route to the Ziegler Defense. After 5. Bc4 exf3 6. Nxf3 Bf5 it reaches a Ziegler line, and this order denies White the 6. Bd3 option. Other White tries are less attractive: 5. Nxe4 leads to an inferior version of the Caro-Kann Fantasy Variation, and 5. fxe4 e5 is good for Black. The rarer replies are 4...Nc6 (Lamb), 4...a6 (Gedult), 4...e3 (Langeheinicke), which returns the pawn to blunt the f-file, and the countergambits 4...c5 (Brombacher) and 4...e5 (Elbert). Against 4...e5, 5. dxe5 attacks the knight, and after 5...Qxd1+ 6. Kxd1 it must retreat to d7 or g8.