For casual OTB games if I think they're below 1400 or so then I play something classical like I'll meet 1.e4 with 1...e5 and 1.d4 with 1...d5 and unless there's an outrageous mistake, I don't try to punish anything until we've developed. This gives them a chance to develop and get a normal position before I crush them. That way the game is a bit more educational for them.
If they're a bit better then I play something off beat because getting an imbalanced position is fun. Non-theory or gambits.
If they're within ~200 points then I'll play my normal repertoire, which is solid stuff, Spanish as both colors, and the Nimzo. (And yes the Spanish is 1.e4 e5, but since I don't play the absolute main line, it's pointless to play it against someone who won't know the moves anyway.)
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Yes this leads some players to underestimate me until we've played 5 or 6 games.
I was wondering if other players do this...
When I play someone for the first time, I play one of the following openings:
If they seem like a beginner:
Danish Gambit
If they seem like a seasoned player:
Bird
For beginners, the Danish immediately clarifies how much they understand about chess, and if they are a complete beginner, it ends the game quickly. For seasoned players, the Bird quickly tells me how good they are...can they equalize?
If they turn out to be good, the Bird often causes them to underestimate me for the second game. This happened with a chess teacher I played recently. I lost the first game with white playing the Bird by steady pressure leading eventually to a lost pawn endgame. Then I won a nice game playing the Slav as black, which apparently he didn't enjoy, as he collected his girlfriend and left after that
.