Black tie - long dress?

[ThisIsSylviaDaisyPouncer]

Ladies of S & B... I need your opinion. I have been invited to a wedding in the US with the dress code black tie (that's all it says). I have been to many black tie events in the UK and always worn a knee length dress ... I've never felt underdressed and do always make an effort with shoes, accessories etc. However the mother of the groom told me that black tie always means a long dress. I don't think she would be offended if I didn't comply (she told me a year ago, plus I am British so they'll probably think I'm kooky, or something). But will I be the only one daringly showing my calves? Any opinions welcome - I don't mind spending the money but would much rather spend it on a new bag than a dress I won't wear again!

traditionally black tie is a long dress but the most recent event I went to had a variety of cocktail dresses (knee length), long dresses and one woman even had a trouser jumpsuit on.

Long or at very least mid calf.

We're going to one later this year. Would a 50s style below knee dress (the petticoat underneath sticky out ones) with a bolero type jacket be acceptable?

God I love black tie! so few chances to amp up the glamour these days. Go full length!

where in the US, and what time of year?

East coast, September. So warmish weather probably but not hot. I actually dislike full length on me, it's not a question of not wanting to look glamorous, I look better in knee length (I got married in a knee length dress!) But I don't hate it, I would wear it if I felt I would be alone in a sea of floor skimmers, if you know what I mean.