Long before there were Nautilus machines and Jane Fonda and the Cambridge Diet, women had to rely on what they put on to reshape their shapes. Sometimes, they put on odd contrivances of distortion and pain beneath their apparel. Other times, shape was built in to the structure of the garment they wore.

Now, three fashion exhibitions in New York, continuing through spring, reveal the shape women used to be in and how they got that way:

At the Fashion Institute of Technology is "The Undercover Story," with more than 600 articles of intimate apparel from the 18th century to the present, including a plain white nightgown worn by Queen Victoria and Gypsy Rose Lee's G-string.

Dresses by Charles Frederick Worth, the English designer who changed the shape of French fashion at the end of the 18th century, are on exhibit at the Museum of the City of New York. The outfits on exhibit were worn by his ritzy American clientele, with names such as Astor, Carnegie, Rockefeller and Vanderbilt.

The clothes of one of the least known, but perhaps most inventive, of all American designers, the late Charles James, are presented in an important, effective exhibition at the Brooklyn Museum, well worth the annoying subway or expensive taxi ride.

At this moment, designers in New York and Paris are on a kick to make clothes with shape. But were they to study these clothes from other eras, and the underpinning to shape them, they might consider the obvious: Despite their beauty, they are clothes that belong to another era.

In the early 1800s, as the Fashion Institute show reveals, women slipped "busks," flat wood pieces much like classroom rulers, into channels down the front of their corsets. (Hand carving those busks as love tokens became a favorite pastime for sailors.) To fluff out the shape of sleeves, goosedown plumpers, which resemble kids' waterwings, were used inside the chemise.

Happily, those body shapers are out of fashion. So are the first corsets with stays from the Symington Corset Manufacturers in England, the petticoats and wired crinolines and the bustles worn to shape the backs of dresses. (In the 1880s you were in good shape if you could balance a tea cup on your bustle.)

Laura Sinderbrand, curator of the show and the director of the Edward C. Blum Design Laboratory at the Fashion Institute, has brought together garments from private and public collections for the exhibition, sponsored by Du Pont. Many of the best items have been borrowed from the Kyoto Costume Museum which, surprisingly, has an extraordinary lingerie collection.

"Western lingerie, like western clothing, is a fairly recent phenomenon in Japan," said Koichi Tsukamoto, president of Wacoal, the Japanese-based company that is one of the three largest producers of women's intimate apparel in the world. Tsukamoto helped the Kyoto Museum develop this collection, he said, because western fashion in Japan is like "a flower cut in mid-stem. We have to study these things to help us understand the roots of western fashion ."

The most drastic shaping came in the 1850s with the tournure corset, which was stiffened with metal or wood and laced as tight as the muscle would allow. "Just how tight a young girl's corset would be laced depended on how ambitious her mother was," said Sinderbrand. "I'm sure that is what the attacks of vapors were all about."

At the turn of the century, women assumed a curious monobosom shape, not unlike a pigeon breast. It was easy enough to achieve with bust enhancers and ruffles. But the museum had to have special mannequins constructed to accomplish the effect properly.

Beautiful lingerie and sleepwear, usually seen by few other than the wearer, gets a proper public viewing at the Fashion Institute. Queen Victoria's nightgown is not only monogrammed but numbered; the numbers permitted an orderly rotation. These quite simple items contrast with the elaborate lingerie of the early 1900s, which included corsets made of brocade, lace, ruffles, rosettes and ball fringe, giving the lingerie a festooned look.

Gallery director Marty Bronson found this exhibit very difficult to mount. "If you are showing clothing, you can hide a lot underneath. And you can achieve the shape you need with dacron and foam padding. With this show, there is no place to hide."

Charles Frederick Worth preferred his American customers to all others, even European royalty. "They queens ask the price. American women never do. They simply say: 'Give me the best, the most beautiful, the most fashionable gown,' " Worth once said.

Those fabulous gowns Worth made for his American clients make up a dazzling costume show at the Museum of the City of New York called "The House of Worth: The Gilded Age in New York." It is an extraordinary view of French fabric at its best, as well as exceptional workmanship. Design students and women and men who like to sew will have a feast studying the construction and draping of the clothes.

The show opens in an 1860 room setting, appropriate in this Fifth Avenue museum that looks like one of the grand houses of the era when "ostentatious display of wealth was seen as the mantle of respectability rather than embarrassing over-indulgence," writes Joanne Olian, curator of the collection, in the museum's magazine. It includes 50 gowns from the museum's own collection. They are in immaculate condition, a compliment to the designer, the wearers and particularly the restorers. Women who bought Worth gowns paid up to $10,000, a fabulous sum in those days, and some gowns were rarely worn twice, particularly during the time of Napoleon III and Empress Euge'nie, when an imperial decree ordered that dresses should be worn to the court only once.

Worth, who was born in England, moved to Paris in 1845, selling fabric first, a far more respectable position than being a dressmaker. Eventually, his designs for Empress Euge'nie heightened his influence and expanded his business so that, at one time, he had as many as 1,200 seamstresses working in his house at 7 rue de la Paix. Henry James and Edith Wharton often dressed their subjects in Worth designs.

The Worth show documents well how Worth changed the shape of women in the Gilded Age, the period between the Civil War and World War I. He is credited with inventing the crinoline (and causing its demise). And some say he created the bustle. Whatever he designed, his followers were madly faithful to his dictates.

The show ends on a depressing note, with Worth's design for the YWCA overseas uniform in 1918. But for those who want a lasting impression of the clothes and their extravagant prices, the gift shop at the museum offers dolls dressed a la Worth for $1,400 to $2,000 each.

Charles James was such a miserable man that if the way a client looked in a gown didn't suit him, he would recall the dress on the pretense of adjusting it and never return it. And if he didn't like what was written about him, he would besiege writer and editor with pages of wrathful prose.

In fact, one museum costume consultant was so leary of James' vengefulness, she established a policy of not exhibiting his creations rather than risk having him literally destroy the display.

And so it has taken till now, five years after James' death, to have a worthy exhibition of his works. Appropriately, it is at the Brooklyn Museum, where James did some original research. That museum now owns the richest collection of James' works.

Writes Michael Botwinick, director of the Brooklyn Museum (who soon will head the Corcoran) in the introduction to the publication accompanying the exhibit, "The Genius of Charles James": "It must fairly be said that this is a project that could not have been realized in James' lifetime. He was too tragically tormented by a private set of demons to have allowed such a thing to come to pass."

James has been called America's most inventive designer, classed with Madame Vionnet, Chanel and Cardin. Christian Dior credited James with inspiring the "New Look," and when Paul Poiret, the Paris couturier, saw James' designs he said, "Charles James, I pass you my crown."

James created dresses and coats with the eye of an architect, perfecting cuts and construction that continue to influence fashion. Both the exhibition and the book contain his remarkable architectural drawings for his fabulous evening dresses.

The women who bought his clothes, including Babe Paley, Austine Hearst, Polly Logan, Dominique de Menil, Millicent Rogers and Jennifer Jones, needed special instruction on how to wear them. Some dresses, built rigidly with the seams as girders, could literally stand on their own. When "Bootsie" Hearst went to the coronation of Edward VII in England, her James gown traveled in a black, coffin-like box.

Women rented rooms at the Waldorf-Astoria and had James' dresses delivered there to be worn to a ball. There was no thought of traveling to the Waldorf wearing one, not even in a limousine.

James cared enormously about the public presentation of his clothes and usually was on hand when they were photographed. That accounts for his close association and friendship with Cecil Beaton, who documented James' work not only on film but with pen and ink as well. The James-Beaton liaison worked brilliantly for them both.

Elizabeth Ann Coleman, curator of costumes and textiles for the Brooklyn Museum, writes in "The Genius of Charles James": "As in sculpture these are not random lines; each was carefully contemplated in the construction of the gown--a mathematical problem solved with calipers and equations. Taken individually, each of the gowns is a strong statement; grouped, an artist's palette is awash with color of the most refined tones. There is no weak link."

In spite of his outrageous personality, James probably was the most realistic of almost any designer. He once said, "There are not many original shapes or silhouettes--only a million variations."

James was a perfectionist in every sense. He supposedly spent three years and $20,000 to perfect a sleeve. "My designs are not luxuries, they represent fashion research," he said.

The exhibition and the book, writes Botwinick, " is our bouquet to him. He, after all, fashioned the flowers; we have merely composed them into an arrangement in the hope of bringing his conception of fashion, line and color into sharper focus."

How brilliantly it succeeds.