What is blackmar-diemer?

Blackmar-Diemer is a chess opening that is characterized by the moves 1.d4 d5 2.e4. It is classified as an aggressive Gambit opening, where White sacrifices a pawn on the second move to gain a lead in development. The opening is named after Armand Blackmar and Emil Diemer, two amateur chess players who popularized the opening in the early 20th century. The opening is considered risky as it puts White on the back foot if Black accepts the gambit pawn. However, it can result in an exciting game with plenty of tactical opportunities. The Blackmar-Diemer opening is not commonly played at the highest levels of chess, but it can be an effective surprise weapon at the club level.