Jenny says: Today, black-tie means that women have the choice of wearing a long dress, a formal cocktail dress or very dressy separates. However, if you want to avoid feeling underdressed, an event's dress code can be determined by the invitation's wording. "Black-tie" traditionally means men wear tuxedos and women wear either full-length gowns or skirts that reach the floor and are paired with elegant blouses. Over the years, black-tie attire has become less formal, but to me, long is still the safest and most appropriate option when attending a black-tie event. "Black-tie optional" allows for more variety and typically means the host is suggesting you dress in your most formal attire, preferably black-tie, but not insisting upon it. A little black dress is completely appropriate for black-tie optional.
Another way to determine proper protocol on when to wear long vs. short dresses is based on the type of event being held. If the invitation indicates the party will be relatively short and will only include a cocktail hour, a simple black dress will suffice. On the other hand, a formal dinner party or a longer event calls for a more formal dress, such as one that reaches the floor.