Chicago has more than a few places to sip cocktails and table talk (The Violet Hour, Billy Sunday, and The Whistler for example), but The Barrelhouse Flat has a distinctive second floor that elicits an almost palpable sexual energy. If Humphrey Bogart were to go on a first date with Mae West, they'd definitely drink cognac-based cocktails at this joint.
The Barrelhouse Flat has a more casual atmosphere than, say, The Violet Hour, but it's not the type of place to rock a salsa-stained Derrick Rose jersey. Dress nicely and, for the love of Based God, don't sip your drink from a straw. You look ridiculous.