Blackmar–Diemer Gambit

Blackmar–Diemer Gambit (or BDG) is one of few gambits available to White after opening with d4:

1. d4 d5
2. e4 dxe4
3. Nc3 ...

White then subsequently makes a move like f3.

The gain is a tempo to White, in giving up a paw, is a partially open f-file.

Grandmasters disfavor this opening and it is almost never used in championship matches.