Ladies' Black Tie

Any black-tie evening would be virtually redundant without the presence of ladies’ attire intended to be set off by their staidly clad escorts.

According to wikipedia, appropriate women's dress for a black-tie affair ranges from the short cocktail dress to the long evening gown as determined by current fashion, local custom, and the occasion's time. 

The 2004 edition of Emily Post's Etiquette simply dictates a "formal evening dress or short, dressy cocktail dress”.  Notably, the author limits the appropriateness of dressy separates for Black Tie Optional.

Letitia Baldrige's New Manners for New Times recommends the following:

• floor-length, short, or three-quarter-length evening gown; evening sandals or pumps; and an evening bag of silk, satin, peau de soie, velvet, or brocade material

• jewelry – same as for cocktail party [sparkly earrings and bracelet or simple button earrings and a very simple gold or silver necklace], only slightly more of it

• a fur, if you have one

But the most elaborate advice is provided by the UK's definitive source of social protocol, Debrett's New Guide to Etiquette and Modern Manners:

For women 'black tie' today is infinitely more complicated, as its presence on a card can signify any level of dress, ranging from a little lycra number to just sub-white-tie levels of splendour.  The old clearly defined distinctions between dinner and dance dresses have largely disappeared and their place has been taken merely by confusion.  Thus it is important to ascertain from the hostess what dress she is expecting.  Good hostesses could follow the lead of a well-known duchess, who sends a written invitation to friends that includes a few short lines dispelling any dress confusion: 'Saturday night is black tie, but palazzo pants will be absolutely fine.’

The invitation itself will also give clues.  A simple drinks party from 6:30 to 8:30 will require only a little black dress, while a more lavish affair to celebrate someone’s twenty-first, which consists of drinks, dinner and dancing, needs a much grander dress.  As an inveterate party-going friend says: ‘The greater the effort of the hostess, the greater the level of formality expected of the guest.’

The choice of long or short dress can also be vexing.  Traditionally long is more formal than short, but these distinctions are fast disappearing . . .  The choice today is largely based on what suits the wearer, her legs and her relative age.  Long is the preferred comfortable option of older women, while young girls invariably feel more suited to short.   However, a long dress or skirt remains the safest option for all, as it is always special, and appropriate to any black tie event grander than a cocktail party.  Best jewels, with plenty of sparklers, can be worn, but obviously tiaras are inappropriate.