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.