It seemed like such a good idea, a modern way to deal with clothes. No rules. Wear what you want. Short hemlines or long, pants or skirts, blue or fuchsia. No difference.

But suddenly 40,000 invitations are in the mail to the Inaugural Ball: white tie, black tie optional. Panic. What does it mean?

Back to the rule books. They help a little, but not completely. Some of the modern ones skip all questions on black tie. Old ones demand long white gloves (now about $150). So what is a person to do?

"Just try to think of it in terms of what Lucille Watson would have worn as the mother in 'Watch on the Rhine,'" says (in his most dulcet tones) New York writer and style arbiter Robert L. Green.

"The inauguration of the president is the democratic equivalent of the crowning of a royal head. Why shouldn't it be treated as a gala and significant moment with parties and dress-up? This is the return to corporate goals. It's appropriate to go back to some formality. It reflects the age of the people involved and for both Nancy and Ronald, childhood fantasies of what was correct, or not correct, taken from Emily Post and Hollywood. . . And I'm all for it."

What will score with the Reagans is basically following the rules, says Green. "So to be thought of as socially acceptable to the new forces, the more conservative the garb, the more familiar the line, the more acceptable. What Nancy Reagan will look for in ladies will be elegant clothing without a degree of costume competition. She's setting up a standard."

And for those who have forgotten the old rules, think Hollywood: Fred Astaire, Audrey Hepburn, Grace Kelly, "Roman Holiday," "Dinner at Eight." Here goes:

White tie, black tie optional: Means white tie clearly preferred for the men, long dresses for women.

"Long dress or a long skirt, for that night, anyway -- that's the least they can do," says Nancy Reynolds, assistant to the director for East Wing transition. White tie means white tie, stiff wing collar and shirt, white pique waistcoat, studs, tailcoat, low-key jewelry (studs and cuff links) and decorations if you come by them legitimately.

Says Letitia Baldrige, social secretary for Jacquelyn Kennedy Onassis when she was first lady and now consultant to Mrs. Reagan: "Most men usually look so splendid in their white tie that the compliments they receive from all the women make up for the discomfort suffered." (From "The Amy Vanderbilt Complete Book of Etiquette," revised and expanded by Baldrige.)

White tie for women: Means putting on your best. Long dresses are essential: It's Mrs. Reagan's party in a way, almost like a wedding, and she sets the tone.

"Dim the glitz," says Green, "and skip the matching emeralds." (Think of it, he adds, as one step above preppie, and one step above the traditional country club.)

Long white gloves: Welcome, if you have them, can borrow or afford to buy.

They look best with strapless dresses, says Nancy Reagan's favorite designer, Adolfo. And remember they come off for dinner and a receiving line.

"Please no dangling extra hands," says designer Bill Blass, referring to that ungraceful business of letting your hand poke through the unbuttoned wrist opening, leaving a dangling glove.

And please no chubby arms stuffed into skinny gloves. And no jewels, rings or bracelets over the gloves.

Tiara: "Leave it home unless you have earned it," says Green, or unless it is part of a famous jewelry collection. "No one wants to upstage the bride . . . or embarrass her."

Top hat: Like long white gloves, another endangered species. But wear it if you have one and don't forget to leave it in the checkroom.

Black tie: Means black tuxedo. Virtually any vintage will do if it is not exaggerated to the point of looking silly.

From Tish Baldrige: It looks right if it is the kind of suit that no one will remember what it looked like the next day. White -- plain or pleated-front -- shirt.

Fancy cummerbunds: Okay (same goes for vest), so long as they don't light up in the dark.

Black-tie occasions: Other than the ball, call for very dressy clothes, but pants or less-than-full-length dresses are acceptable.

Other thoughts prompted by the questions that have been pouring in:

Shoulder bags: Handy to leave your hands free for dancing or for fighting the crowd. But if you have a pocket for your lipstick and claim ticket, forget the bag.

Shoes: Think pretty and comfort when choosing. It may be a long, cold walk from the place you park. That doesn't countenance saddle shoes or slippers, but pinched feet won't add to the evening memories.

Coats: Where we depart from the rules. Dress warmly. Leave your coat in the trunk of your car if you park inside; or wear one you don't mind getting crushed in a crowded checkroom or stashing under a chair.

Don't worry about what your coat looks like: You'll never meet Mrs. Reagan in the garage.