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 trades the e-pawn for an extra tempo of development and open lines toward Black's king. The pawn usually comes back once Black castles carelessly or grabs more material; the real currency is tempo.

• The whole point of 2. e4 and 3. Nc3/4. f3 is time: 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; 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; many club-level White players feel satisfied just getting the pawn back and start playing passively, so keep bringing pieces forward for compensation through piece activity.
• 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.