Although evening gowns are standard fare for black tie weddings, you can get away with a formal cocktail dress (of the longer variety) or even an elegant suit. (brides.com)
Or you can wear long gowns for a formal black tie wedding. (annchovie.com)
I wore it to a fall black tie wedding and got nothing but compliments - perfect alternative to the basic black formal dress. (renttherunway.com)