The 20th century was the era of the office. The 21st century . . . well, who knows?

But for now, the daily lives of the majority of working Americans are lived in offices. "On the Job: Design and the American Office," an exhibition opening today at the National Building Museum, takes a long look back at the evolution of the lowly cubicle.

The exhibition is a typical contemporary media layer cake--photographs, lots of words in explanatory captions, excerpts from films and television shows, family-friendly interactive stations and computerized versions of almost everything in the show.

But its stars are what might be called real-time objects--candlestick telephones of a century ago and the latest miniature, clunky typewriters made by Civil War gun manufacturers and sleek but equally outmoded electrics, "scientific" office manuals that go into incredible detail to ensure the "right" office routine, all sorts of period stuff from ashtrays to water coolers, and, most important, a select group of original desks and chairs that, by themselves, pretty well sum up the history of office design.

A lot of office workers may scoff at the notion that their workplaces are exemplars of anything as sophisticated as "design"--and with good reason. The average American office is duller than dishwater, and probably not much healthier--row after row of identical "workstations" made of cheap "systems furniture" in airtight buildings of scant aesthetic merit.

Nonetheless, architects and designers can't be blamed--at least not too much--for bottom-line aesthetics. At their best, the design professions have helped us to envision workplaces that are more both more productive and more humane than the sorry norm.

Before the Industrial Revolution, offices were reserved for the use of a few professions, and mixed in with everything else--markets, stores, warehouses, artisans' shops. The growth of capitalism--and especially its spread across the United States--produced new financial and managerial classes, and created thousands (ultimately millions) of new, specialized jobs.

Writer Upton Sinclair, in 1919, characterized the new clerical workforce as "white collar"--a phrase that quickly entered the national lexicon. Working conditions for many of these new clerks--a large percentage of them women right from the start--were not that much better than those of their "blue collar" counterparts in the factories (although clerks were much slower to join unions).

Early in the 20th century, the Progressive Era called forth a paternalistic response--the "industrial betterment" movement pioneered by a few enlightened capitalists, including the owners of the Larkin Co., a mail-order soap outfit in Buffalo, NY. In 1902 these folks hired the young Frank Lloyd Wright to design a new headquarters--and thereby spawned one of the century's most innovative and influential office buildings.

Wright responded empathetically to the company's uplifting themes. His Larkin Building, completed in 1906, was replete with evangelical inscriptions and amenities that included a library, a classroom, recreational facilities and even a huge pipe organ high in its towering atrium. The architecture of that light-filled atrium was in itself uplifting--the building anticipated by many decades such latter-day workplace triumphs as Kevin Roche's 1967 Ford Foundation Headquarters in New York and Lord Norman Foster's 1997 Commerzbank tower in Frankfurt.

Wright's building, featured in the exhibition in more than a dozen photographs, a scale model and an excellent, eight-minute computerized tour developed at the University of Virginia, was ahead of its time in many respects. Its filtered air system was a marvel; its layout looked forward to the today's "open office"; and its built-in furniture antedated the contemporary workstation by half a century.

However, the building was not very adaptable to other uses. After the Larkin Co. failed, the structure was abandoned and then torn down in 1950. (On the other hand, perhaps it was people who were not imaginative enough to conceive new uses for the masterpiece.)

The coming of the open office was the century's most significant single change in the field. It happened in stages after World War II. Before then, fully partitioned offices with exterior views were the norm (as they remain today on executive floors and for some entire professions--lawyers most notably), with leftover areas designated for the "secretarial pool." Afterward--especially in the '60s--corporate hierarchies were called in question and "enhanced communication" among office workers became a corporate goal.

Financial incentives also played a role. Shows at the Building Museum often reveal arcane but telling facts such as this: In 1962 Congress established a 10 percent investment credit on personal property that applied to such things as movable partitions, but not to fixed walls. Cheaper to build in any case, open offices with their systems furniture--sort of a cross between furniture and architecture, with the modular desktops, drawers, partitions, shelves and such--got a further boost from the law.

Systems furniture actually can be entertainingly clever and quite handsome. Witness, in this show, the 1964 Action Office, designed by George Nelson with inventor Robert Probst for the Herman Miller Co. With its interchangeable parts, sleek cantilevers, die-cast aluminum legs and stool-like chair, it sums up the future of office furniture for the next three or four decades, at least.

The Action Office also demonstrated that good looks and efficiency are not contradictory although, ironically, the design proved a bit too adventurous for the marketplace. The system did not sell well until the Miller company introduced a tamed-down revision four years later.

In terms of sheer acreage, open offices still predominate but--happily for design fans--the electronic revolution has stimulated some fresh thought on the office front. The average office building and the spaces it encloses may still be painfully unimaginative, but the new energy of the Silicon Valleys of the world is being expressed in a variety of new shapes and spaces.

Flexibility, creativity and interaction are among the new watchwords--the idea is to design both spaces and furniture that can be changed easily and quickly, to accommodate a changing workforce and to encourage informal--or even formal--collaborations. This has produced enough good work--buildings and spaces designed by Studios Architecture for Silicon Graphics in California stand out in this show--that one can almost conclude that it isn't simply a fad.

And then comes the big question about whether the office as we have known it--an actual place where millions of face-to-face (F2F in computer jargon) meetings take place every day--will survive. Or will a tremendous decentralization prevail, with everyone working at home or in the coffee shop or--as one familiar advertising image has it--on the mountaintop? Will we all go our separate ways, linked in virtual space?

Because the questions are more or less unanswerable, exhibition curators Donald Albrecht and Chrysanthe B. Broikos make an educated guess that though offices may be modified, they won't go away. "It seems likely," they say, "that people will need human contact and the social cohesion of the office's physical space to be productive."

Glad to hear it, even though the design of most offices leaves lots and lots to be desired. The exhibition continues through April 29, at the National Building Museum 401 F St. NW.