against the black kingside. The best way to carry out this approach is via 6.Bf4, as 6.Bg5 (as played by Bogoljubov in his game against Diemer) is well met by 6...Bg7 7.Qd2 0-0 8.0-0-0 c5!, when Black stands better. If Black tries the same approach against 6.Bf4, i.e. 6...Bg7 7.Qd2 8.0-0-0 c5, then 9.d5 a6 10.d6! gives White good chances. Ziegler Defence: 5...c6 Black's most critical response to the Blackmar–Diemer Gambit is 5...c6, known as the Ziegler Defence due to Diemer's tendency to name lines after opponents that first played them against him, but most of the theory of the line was established by Gerhart Gunderam, who advocated 5...Bf5. Most modern authors recommend this as Black's antidote to the BDG, sometimes via O'Kelly's move order 4...c6. The old main line runs 6.Bc4 Bf5 7.0-0 e6 8.Ne5, when Black should avoid 8...Bxc2?! 9.Nxf7!, but instead play 8...Bg6!, when White ends up with very little to show for the lost pawn. More dangerous for Black is 8.Ng5, the Alchemy Variation, where Black has to be careful not to fall for various sacrifices on e6 and f7, but White probably does not get enough compensation for the pawn after 8...Bg6 9.Ne2 Bd6. German FIDE master Stefan Bücker regards Black as clearly better after 10.Nf4 Bxf4 11.Bxf4 0-0, but Christoph Scheerer believes that White can generate attacking chances with 12.c3 h6 13.Qg4!?. In view of White's problems proving compensation in these lines, ChessCafe.com reviewer Carsten Hansen concluded, "despite all the smoke and mirrors, the Blackmar–Diemer Gambit still isn't viable beyond club-level or rapid-play games". However, Lev Gutman proposed the alternative 7.Bg5 e6 8.Nh4!? Bg6 9.Nxg6 hxg6 10.Qd3, intending to castle queenside and tie Black down to the f7-pawn, promising long-term positional compensation for the pawn. There are currently insufficient practical tests to determine whether it amounts to enough compensation for the lost pawn. Black cannot easily deviate from this line, since after 7...Nbd7 White continues 8.Qe2 e6 9.0-0-0, aiming to launch a strong attack down the e- and f-files, and if 9...Bb4 then 10.d5!. If White tries to enter this setup after 7...e6 8.Qe2, however, then 8...Bb4! prevents White from safely castling queenside, leaving White with insufficient compensation for the pawn. White also has the dangerous, though probably objectively insufficient, second pawn sacrifice 7.g4, analysed extensively by Stefan Bücker. In the 5...c6 move order White has the alternative 6.Bd3, usually intending to sacrifice a