to decline the gambit on the second move with 2...e6 (leading to a French Defence) or 2...c6 (leading to a Caro–Kann Defence), although doing so does not eliminate White's ability to offer alternative gambits such as the Diemer–Duhm Gambit (2...e6 3.c4) or the Alapin–Diemer Gambit (2...e6 3.Be3), or for instance 2...c6 3.Nc3 dxe4 4.f3 or 4.Bc4 intending 5.f3. Main line After 1.d4 d5 2.e4 dxe4 3.Nc3 Nf6 4.f3 exf3 5.Nxf3, Black has five main options: Gunderam Defence: 5...Bf5 The line 5...Bf5 (along with most of the ...c6/...Bf5 defences for Black in general) was extensively analysed by Gerhart Gunderam, who published his analysis in a book Blackmar–Diemer Gambit in 1984. The main response for White is 6.Ne5, intending to attack the black bishop with an advance of the kingside pawns and, if appropriate, weaken Black's kingside pawn structure with Ne5xBg6. Black can respond with 6...e6, when after 7.g4, 7...Be4 leads to tremendous complications, e.g. after 8.Nxe4 Nxe4 9.Qf3 Qxd4 10.Qxf7+ Kd8 11.Qf4. More common is 7...Bg6, which leads to quieter play, when White's best response is probably 8.Bg2 c6 9.h4, with a sustained kingside initiative in return for the pawn. However, Black also has the option 6...c6 intending 7.g4 Be6, when White has to play accurately to prove enough compensation for the pawn after 8.g5 Nd5 or 8.Bc4 Nd5 9.Qe2 Nd7. White has an alternative in 6.Bd3, directly challenging the bishop, but Christoph Scheerer doubts that White gets enough compensation after 6...Bxd3 7.Qxd3 c6 intending ...e6, ...Nbd7, ...Be7 and ...0-0 with a solid position. Teichmann Defence: 5...Bg4 The move 5...Bg4 pins the knight on f3, often with the intention of swapping it off and undermining White's central control. White's best response is to attack the bishop immediately with 6.h3, when play often continues 6...Bxf3 7.Qxf3 c6 (but not 7...Nc6, when 8.Bb5 is good for White). In this position, White can defend the attacked d-pawn with 8.Qf2 (the Ciesielski Variation), but this allows Black an easy game by preparing ...e7–e5, e.g. after 8...Nbd7 9.Bd3 e5. Alternatively, 8.Be3 is the Classical Variation, where White aims for a slow buildup to a kingside offensive. White's other main alternative is 8.g4!?, the Seidel–Hall Attack, where White is happy to sacrifice the d-pawn in order to gain an increased initiative on the kingside, e.g. after 8...Qxd4 9.Be3 Qe5 10.0-0-0 e6 11.g5. Black can decline the pawn, e.g. after 8...e6 9.g5 Nd5 10.Bd3, leading to sharp play.