Black tie dinner - what to wear

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Help - I never get invited to these things. Not really sure what black tie means in terms of women. Does it have to be long?
I'm partly really excited but panicking about what to wear. I'm not at all a dressy person and I have nothing beyond long skirt/top combinations. But I do want to look the part. I can shop but on a low budget and would ideally want something I can wear again. Size too different to borrow from my friends. (I'm 5'2'', 14).
Any tips / ideas / help greatly welcome.

The BT event I went to mainly was long dresses, but by the time we were sat at a table, no-one knew how long your dress was - and we spent most of the dinner sat down! Mind you, women were in the distinct minority, mainly due to the area.

Is it a regular do? Can you find photos of last year?

Can't really help with dresses, but I was directed to coast and phase eight.

Ooh last years photos good idea, will have a search. Think it will be mainly a sit down dinner but there may be a bit of mingling beforehand.
Thank you - I'm trying to get some idea of what I'm looking for quickly while the sales are still on.