If Black Tie is mentioned on the invitation, women may either wear a long or a short evening gown. Trouser suits are acceptable too, but only if they are extremely smart and elegant.

The dress needn't be as formal as White Tie, so you could choose a trendier style. You should wear nice jewelry and maybe evening gloves.

Of course, Black Tie differs slightly from country to country. Therefore, it is probably the best choice to inform yourself and according to the social and occupational background of your host.