Blackmar-Diemer Gambit: Lemberger Countergambit, Lange Gambit

The Blackmar-Diemer Gambit: Lemberger Countergambit, Lange Gambit is a chess opening classified under ECO code D00 in the Closed & Semi-Closed Games family, arising from 1.d4 d5, including the Queen's Gambit, Slav Defense, and related systems. It is reached after 4 moves: 1. d4 d5 2. e4 dxe4 3. Nc3 e5 4. Nxe4. The resulting position leaves black to move.