Blackmar-Diemer Gambit

White offers a pawn on move 2 to blow the game open before Black finishes developing: after 2...dxe4 3. Nc3 Nf6 4. f3, White is trading the e-pawn for a full extra tempo of development and open lines toward Black's king. The pawn usually comes back on its own once Black castles carelessly or grabs more material, so the real currency of this opening is speed, not material.

• The whole point of 2. e4 and 3. Nc3/4. f3 is time, not material: after Black recaptures on e4, get every piece out fast (Bc4 or Bg5, Qd2 or Qf3, O-O-O) instead of playing slow safety moves like c3 or Be2 that hand the tempo back.
• If Black declines and just holds the pawn with moves like the Weinsbach (4...e6) or O'Kelly (4...c6) defenses, don't force anything — most club opponents who decline are only trying to survive, so keep developing normally and you'll usually get the attack for free anyway.
• Watch for the Lemberger Countergambit (2...dxe4 3. Nc3 e5!?): grabbing the pawn back with 4. dxe5?? runs into 4...Qxd1+, trading queens and killing your whole attacking idea, so answer with 4. Nxe4 or 4. Be3 to keep queens on the board.
• In the main Teichmann line (5...Bg4 pinning your knight), play 6. h3! right away — after 6...Bxf3 7. Qxf3 you've won the bishop pair and put your queen on a square already eyeing f7 and b7, which is a huge practical edge under 1400.
• Once you reach ...c6 and Be3, the plan is O-O-O, Kb1, and Rhe1 or Ne5 — a lot of club-level White players feel satisfied just getting the pawn back and start playing passively; keep bringing pieces forward, because the compensation is piece activity, not material count.
• Black's most common mistake at this level is grabbing extra pawns (...Qxd4, ...b5, ...c5) while still uncastled — look for Bc4 combined with Ng5 or Ne5 hitting f7, and don't hesitate to give up more material if it cracks open the king.