Blackmar-Diemer Gambit

The Blackmar-Diemer Gambit, often shortened to BDG, arises when White answers 1. d4 d5 with 2. e4, giving up the e-pawn to open lines. After 2...dxe4 White normally plays 3. Nc3 and follows with f3, so that after ...exf3 the f-file is half-open and White has gained a tempo for the pawn. It is one of the few genuine gambits available to White after 1. d4, and it can also be reached from the Scandinavian Defense by 1. e4 d5 2. d4. Black is not obliged to capture: 2...e6 enters the French Defense, 2...c6 the Caro-Kann Defense and 2...Nc6 the Nimzowitsch Defense, while 2...e5 is the Beyer Gambit.

The opening grew out of the Blackmar Gambit, 2...dxe4 3. f3, promoted and first analyzed by the New Orleans player Armand Blackmar in the late 19th century. That version faded because 3...e5 gives Black a good game. In 1889 Ignatz von Popiel introduced 3. Nc3, intending 3...Nf6 4. Bg5 (the von Popiel Gambit), and also examined the Lemberger Countergambit, 3. Nc3 e5. The modern form is owed to the German master Emil Josef Diemer, who championed 3. Nc3 Nf6 4. f3; here 4...e5 is ineffective because 5. dxe5 attacks the knight and 5...Qxd1+ 6. Kxd1 forces it back to d7 or g8. Diemer's book of the late 1950s concentrated on the Ryder Gambit, 4...exf3 5. Qxf3, which offers a second pawn.

The main line runs 4...exf3 5. Nxf3, after which Black chooses between 5...Bf5 (the Gunderam Defense, usually met by 6. Ne5 and a kingside pawn advance against the bishop), 5...Bg4 (the Teichmann Defense, answered by 6. h3), 5...e6 (the Euwe Defense, a French-type setup a pawn up), 5...g6 (the Bogoljubov Defense, aiming ...c5 against d4) and 5...c6 (the Ziegler Defense). The Ziegler is widely regarded as the most testing reply and can also be reached through the O'Kelly order 4...c6. Earlier deviations include 3...Bf5 (Zeller Defense), 3...f5 (Netherlands Variation) and 3...c5 (Reversed Albin Countergambit), while White can choose 3. Bc4 (Fritz Attack) or 3. Be3 (Diemer-Rosenberg Attack).

The gambit's soundness is disputed. Theory gives Black several ways to equalize, the engine assessment slightly favors Black, and the opening is almost absent from elite practice, yet it is popular among club players and White scores marginally better than Black in online games. Commentators such as Boris Avrukh and Joe Gallagher have warned that White's initiative can become dangerous and that Black's inaccuracies are punished quickly, so accepting the pawn demands precise defense; Willy Hendriks observed that the main line carries the same engine evaluation as the King's Gambit.