A Black Tie invitation calls for formal attire. Men wear tuxedos, women wear cocktail, long dresses or dressy evening separates. “Formal” usually means the same as Black Tie, but in some trendier cities like New York or Los Angeles, it could mean a black shirt, no tie with a tux. "Black Tie Optional" gives you the option of wearing a tuxedo. A dark suit and tie would be your other option.