Royal Wedding: Proper Guest Attire for Prince William and Kate's Big Day

VIDEO: Nick Watt explains the traditions and expectations behind royal-wedding
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There has been endless speculation about what princess-to-be Kate Middleton will wear on the big day.

But what about the guests?

On Friday, 1,900 lucky guests will convene in Westminster Abbey for the royal nuptials .

Determining what to wear to such a high-end, globally watched affair is no easy matter.

Indeed, some have called dressing for the royal wedding a fashion minefield where one wrong step can -- and will -- land an attendee on a worst-dressed list for years to come.

ABC News went to high-end London-based department store Selfridges for help with the matter.

Because the wedding will take place at Westminster Abbey, leading Selfridges stylists said, sleeves for women are a "do" and looking too sexy is a "don't." Exposing shoulders in a sacred house of worship is considered improper.

And while it's important to look proper, it's ill-advised to appear stuffy. Too many frills or too high a collar can be considered fashion-backward.

Watch a special "20/20" today at 8 p.m. ET for a behind-the-scenes look at the life that awaits Kate Middleton, and tune in again at 4 a.m. Friday for ABC News' live coverage of the royal wedding with Diane Sawyer and Barbara Walters.

Attire for Men

The rules for what male guests should wear to the wedding are quite precise. According to the royal wedding invitation, men are required to wear one of three kinds of dress Friday: a military uniform, a morning coat or a lounge suit.

A lounge suit isn't exactly as casual as it may sound; it means a coat and tie. As for the morning coat, it's a coat with the front cut away-so that just the back part hangs down in the back. The coats come from the days when men rode horses and wore top hats.

Not Just About the Suit and Dress

But royal wedding attire isn't just about the proper suit or dress; it's also about the accessories. Dents Gloves, for instance, are synonymous with fine handwear in Britain and are a staple of the royal family.

For three centuries, the gloves -- lovingly hand-stitched at a factory in Wiltshire -- have been worn by kings, queens and attendees of royal weddings.