It used to be when you got a party invitation the rules of what to wear were strict and easy to follow. But now it's clear that today's dress requirements are odd and ambiguous. What is a party-goer to do? Here is a glossary of the most common dress dictates:

Casual - This is one of the vaguest instructions found on an invitation. According to Peggy Post, author of "Etiquette" (Harper Collins), how you define "casual" depends on whether you're in a city, the country, or the suburbs, as well as on the occasion and your friends. "In the city, it could be a business suit or sport coat and tie for a man and a pantsuit or skirt or slacks for a woman, says Post, granddaughter of famed etiquette arbiter Emily Post. "It's fine to ask if you're stumped."

Informal - This is almost interchangeable with "casual", except it sounds dressier.

Business Attire - When the party begins right after work, you obviously must wear what you wore to the office.

Fancy Dress - For a woman, as cocktail dress is right. A man might wear his best suit, but not a tuxedo.

Festive Business Attire - Talk about nebulous: does the host expect a female guest to wear a suit with a lace bra underneath? The Boston Lyric Opera asked for this dress at it's April Corporate Night at the Opera benefit. "We wanted an alternative to black tie," says the BLO's Patricia Wanty. "What we meant was for the women, a sequined top or satin jacket or ruffles, lace, and big baubles. For the men, bow ties, colorful ties, or your Christmas party suit, dressier than your everyday business suit."

Wear White - The perpetrators of the party want a color theme so it would be good of you to play along.

Black Tie - (also known as "formal"). These rules, thankfully, are still rigid. For men, it means specific formal clothes, a dinner jacket, nowadays commonly called a tuxedo, a black jacket, white formal shirt, black bow tie, and a cummerbund. For women, it now means "a dress or attractive evening suit of any length or style, including tuxedo pants, fancy culottes, or pajama-style trousers."

Black Tie Optional - It means the party givers would like you to wear black tie but they won't push you.

Creative Black Tie - Men, this is your chance to bring out all those strange bow ties you've stuck at the back of your closet .. the ones adorned with naked women, peonies ... whatever. Women, wear what is too wild for normal black tie events.

White Tie - Not only must men wear a white bow tie, but they must add a white pique waistcoat and black tailcoat to their black pants, black patent shoes, and stiff wing-collar shirt. For women, pull out all the stops, including a long gown, gloves, and your best jewelery.