I saw this answered in a post from 2009. Didn't know if it was still considered "vulgar" to leave a button undone on your jacket coat sleeve?
Functional Buttons...leaving one unbuttoned
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I wouldn't go so far as "vulgar", but it is definitely not SF-approved. I generally do up all the sleeve buttons, but on occasion I will choose to leave one unbuttoned just because I damn well feel like it. I suspect a very small percentage of real world inhabitants would notice or care. And for those few who might notice and choose to scoff, well, Rhett Butler had a good line for that.