''We all want to fit into the accepted visual parameters of the time,'' says Barbara Dickstein, a New York City garment district graduate who is now the Smithsonian's 20th-century fashion expert, ''but we all want to be a little bit different.''
The times and fashions she tends to talk about have much broader parameters than usual, she thinks. An easy way to visualize the dramatic switches over the century is to think of women's silhouettes - the Gibson girl of the 1890s, the flapper of the 1920s, the ''new look'' (pinched waist, full skirt) of the 1950s, the ''dress-for-success'' look of the 1970s.
''Now we're sort of all over the place,'' says Mrs. Dickstein in an accent that still says New York. ''For the first time, women have more than one silhouette, along with the option of wearing pants and any length skirt.''
The ideal figure of this century, she says, is much different from times gone by: ''It's thin, which says you take care of your body, you jog, you play tennis , you take aerobics.''
She wonders a little, however, if the ''fitness'' craze that seems to be taking over the country isn't just a way to achieve the thin look. ''I wonder whether in the centuries to come they'll look back at us and think, Why were all those people running around on such hard pavement, in all kinds of weather? Wasn't that ridiculous?''
Citing a theory by architect and writer Bernard Rudofsky, she notes that human beings have done all sorts of things over the centuries to manipulate their bodies into the ideal fashionable shape - forehead sloping among various tribes, feet binding among the Chinese, and pointed toe shoes among Western 20 th-century women.
One of the major factors behind the changes in women's fashion, Mrs. Dickstein believes, is ''the changes in women's lives. At the beginning of the century, she began to go out more, she joined clubs, she had fewer children.'' That's the point where you see ''the tailored suit and the walking suit,'' items that look hauntingly familiar to today's office women.
Women haven't stayed in sensible-looking clothing throughout the whole century, though. ''It's more like a back and forth,'' Mrs. Dickstein explains, drawing a star pattern on a legal pad in her office at the National Museum of American History.
Today, ''most working women want to build up a wardrobe of reliable pieces they can mix and match.''
Mrs. Dickstein watches those women with interest. ''I have a friend who became a lawyer in an all-male firm,'' she says, ''and when she started I called her up and said, What did you wear to work?''
This was during the dress-for-success phase when ''women didn't want to stick out any more than they had to,'' so the woman wore a suit - and Mrs. Dickstein collected it for the Smithsonian. ''Now,'' she reports, ''she's much more relaxed about it and wears a softer look - things like pretty dresses.''
The suiting-up era was an effort ''to be sensible about clothes,'' she thinks. ''Men had been allowed to be sensible, to wear sensible things, and now women wanted it too.'' She sees this extending to shoes - outside the office.
''Those women who walk to work in tennis shoes and then change at the office fascinate me,'' she says. ''It's such a difference. In the 1880s, when you went out in public, you dressed up because you were known in public and would be recognized. Now,'' she says, ''women can wear tennis shoes outside on their way to work where they're sort of anonymous, but once they get to the office, what do they do? Dress up again.''
Frivolous, fun clothes still show up on the backs of working women ''at night and on weekends,'' she says. Swimsuits, evening clothes, and weekend gear are all ''very different from office wear,'' she points out.
''We like to think as women that we have a lot of choices,'' she concludes, ''and our fashion reflects this.''