"Good, party-line modern architecture, that's what we did when we got out of school," George Hartman is saying.

"At this stage it's hard even to think about architecture school," responds Warren Cox, the other half of Hartman-Cox, Washington's award-winningest architectural firm. "I mean, that was a long time ago."

The most gratifying of their prizes has just arrived -- this year's Architectural Firm Award from the American Institute of Architects, to be presented formally in May at the AIA convention in New York.

Hartman, 51, and Cox, 52, are the first Washington architects to receive this honor since it was initiated 26 years ago. "We're lucky to get the firm award now," Cox says, reflecting on the unfashionable stylistic diversity of their recent work. "In another two years or so, we'd never get it."

Hartman and Cox have been together professionally since the early 1960s when, fresh out of school -- Hartman, bachelor's and master's degrees, Princeton, 1957, 1960; Cox, ditto, Yale, 1957, 1961 -- they came their separate ways to work for the Washington firm of Keyes, Lethbridge and Condon.

They became Hartman-Cox, the partnership, in 1965. The evolution since then of their ideas in the form of houses, dormitories, churches, embassies, libraries and, of course, office buildings large and small has been one of the more fascinating episodes in the history of Washington architecture.

It is a story with a strong local accent that reflects and in some ways anticipates a national move away from "party line" modernism, a story of starts and stops, advances and retreats, of abundant combinations and transformations -- sometimes brilliant, frequently ingenious, often witty, generally tactful, almost always incisive.

To label the firm's work remains difficult. As the AIA Jury on Institute Honors pointed out, "No two projects are the same because each is a response to the specifics of site, program and context," and such responsiveness is indeed one of the clear consistencies across 23 years. As much as any firm in the country this one has broadened the concept of contextual architecture, helping redefine what it means to add new pieces to an ever-changing city.

"Urban design considerations are more important to us than architecture," Hartman summarizes. "The city is more important than the building."

Architects as a rule are not given to such self-effacement. Fortunately, in the actual buildings, this Hartman-Cox theme has been interpreted with a great deal of spirit. The changes in their style, though gradual, have been profound, and today it is possible with some certainty to divide the work into early, middle and late (or recent) periods.

The early period lasts to the middle '70s, and is characterized by buildings that are abstract and site-specific; the middle, transitional period, continuing to 1980 or so, sees the firm struggling to balance ideas of historical imagery with modernist notions of "honest" structural expression; in the work of the last five or six years one witnesses the emergence of a thoroughly eclectic approach to style.

It's a treat that this Hartman-Cox evolution can conveniently be traced in a day's tour. Only a handful of the firm's major commissions are located outside the Beltway -- an impressive handful, to be sure, including the National Humanities Center in Raleigh, N.C. (completed in 1978), the U.S. Embassy in Kuala Lumpur, Malaysia (1986), a headquarters complex for a big grocery firm in San Antonio (1987), additions to Monroe Hall at the University of Virginia in Charlottesville (1987) and the Chrysler Museum in Norfolk (under construction). But Washington has been the proving ground.

Among the early nonresidential buildings none are more memorable than the chapel on the Mount Vernon College campus at Foxhall Road (1969) and the Euram Building facing Dupont Circle (1971). The chapel, situated on a wooded lot and in a sharp ravine, has a knife-edge sloping roof; it's a bold piece of geometry but it looks as if it belongs and not, as does the crystalline Hartman-Cox dormitory nearby (1970), as if it just landed on the planet. The chapel's best aspect, though, is the interior space, a clean-lined work of abstract art designed to frame and amplify changes in natural light -- which it does, splendidly.

The Euram Building, too, is a walk-in work of sculpture, sheltering one of Washington's more shadowy, and oddly appealing, interior courts. Cox says they were thinking of the "brutalist" forms of Paul Rudolph, one of his famous Yale mentors, and the striking future-is-now 1960s architecture of Japan's Kenzo Tange. This is apparent, but Hartman and Cox were wearing Washington shoes at the same time -- the Euram is perhaps the city's most civilized modern office building. When at their best in the modernist idiom, Hartman and Cox were by no means party-line hacks.

Key transitional pieces include the National Permanent Building (1977) at 18th Street and Pennsylvania Avenue NW, and additions to the Immanuel Presbyterian Church in McLean (1980). National Permanent is perhaps the loudest of all Hartman-Cox structures -- it confronts its prominent corner like the simultaneous clash of a hundred cymbals. The architects say they were taking their cues from Alfred B. Mullett's nearby Old Executive Office Building, a telling comparison because they were attempting to emulate the rhythmic effects of Mullett's 900 Doric columns without using classical orders. With its facades enlivened by structural columns and exposed air ducts, National Permanent is abstract art on the very edge of representation.

At Immanuel Presbyterian Church Hartman-Cox crossed over this edge -- hardly ever to return, as it happens. With its pitched roofs, dormer windows and white-painted board-and-batten walls, the new church hall is every inch the image of 19th-century country church or 18th-century meeting house -- a dream image, almost, sleek and pure, except for the jarring note of two giant diamond-shaped windows under the highest eaves. "We wouldn't do those windows again," Hartman says.

Today, the firm's embrace of history is self-confident, full-blown. Critical to the transition was a seven-year involvement (1975-82) with the Folger Shakespeare Library on Capitol Hill, Paul Cret's refined essay in stripped classicism. "It was the first time we'd worked with a really fantastically detailed building," Cox recalls, "and it jumped the level of design detailing in our office dramatically."

It also confirmed for both architects the new direction they were taking. By the early 1980s, when the Folger renovations were complete and work had begun on designing a new reading room, Hartman and Cox had discovered that they could design in almost any style -- something they had been taught not to do at school. The Folger addition is in places self-conscious and awkward -- the fluted exterior "columns" are just marble slabs pasted on a steel frame. In others it's intentionally tense (as John Morris Dixon has pointed out) -- the rusticated "stonework" inside the vaulted chamber is nothing more than tinted plaster. But the net effect is one of classical composure. It's a quieting space, one of a very few truly memorable public rooms created in Washington since World War II.

The firm has prospered in the '80s. Its commissions, completed, under construction and in design, include some of the city's biggest and most prominent buildings: 4250 Connecticut Ave. NW, a big piece that proves Hartman-Cox can still do terrific modern architecture when it seems fitting; the Sumner School project (1985) on M between 16th and 17th streets NW, in itself an encyclopedia of styles and approaches to combining new with old; the gigantic structure at 1001 Pennsylvania Ave. NW, a building of such convincing old-fashioned elegance that it's been mistakenly identified as a restoration.

"That doesn't bother me at all," Hartman says. "We mean to be one set of architects who can do things in 10 to 15 different styles," an attitude that links Hartman-Cox not so much to other contemporaries who, likewise, have rejected their modernist roots, but to an earlier generation of 20th-century eclectics, to John Russell Pope, perhaps, or Charles Platt, or Waddy B. Wood or George Oakley Totten.

It's an attitude sure to be popular but not likely to generate much of a professional following -- "I just wish George and Warren were still out there leading the way," admitted one disappointed admirer. Nor, obviously, is it the only approach to contemporary architectural and urban issues. But the difficult trick is to do it very, very well -- precisely what Hartman and Cox, and their principal colleagues Mario Boiardi, Lee Becker, Graham Davidson, Steve Vance and Ankie Barnes, have done.