When Pierre L'Enfant made his great plan for Washington he established a strong symbolic substructure, the diagonal avenues representing the authority and ideals of the new national government, and the orthogonal grid of streets denoting the everyday life of the city. The plan also guaranteed a whole bunch of odd-shaped building lots where the avenues meet the streets, a design challenge that over two centuries has given Washington much of its rich architectural character.

Not often, however, does an original response to the odd-lot condition come along. For this reason alone the new Cato Institute building at 1000 Massachusetts Ave. NW is cause for celebration. Not simply a clever solution to an aesthetic problem, it literally embodies the contrasts of the L'Enfant plan in three-dimensional form. Plus it's pretty, a little jewel.

Dusk is the best time to view this building, and the northeast corner of 10th Street and Massachusetts the best place to stand. From that position at that time a viewer can easily distinguish the building's basic geometries: A cube of clear glass and white-painted steel, its main wall parallel to the diagonal avenue, partially frames a cubical masonry building, its walls parallel to the gridded streets. On bright days the glass walls, reflecting sun and sky, lose something of their transparency; at night, the masonry building disappears. At dusk the view is enchanting and crisp: The glass structure glows softly and the masonry walls are bathed in subtle light.

This is quite a visual-intellectual punch for so small a building. Happily, there is more.

Charles George, the leading design architect on the project for the Washington office of Hellmuth, Obata & Kassabaum (though he's no longer with the firm), recalls that, besides reflecting the site's geometry, the design had two other sources. One was the sometimes acrimonious give-and-take among the client, city planners and neighborhood groups. The other had to do with the unique makeup of the Cato Insitute.

As an office building in an area zoned for a commercial-residential mix, the project was subject to extensive review as an exception to the rule. Neighborhood reaction to an initial design (by another HOK team) for a precast concrete structure was understandably hostile. All the same, institute President Edward H. Crane continued to insist on a modernist building, perhaps a surprising choice for a think tank whose favorite philosopher may be Thomas Jefferson. "We believe that the ideas we promote are modern ideas," Crane explains, "and that it is important to reflect the fact that society is dynamic and changing."

In the process of design, George understandably became intrigued with the Cato Institute. Named after a Roman philosopher and opponent of Julius Caesar, founded in San Francisco in 1977, headquartered in a Capitol Hill row house since 1981, the institute in its publications and policy pronouncements reflects an unorthodox mix of free-market economics and libertarian social views. "I wanted the building to capture something of these possibly conflicting and contrasting philosophies," George says.

The architectural resolution of such diverse conflicts began with a brilliantly simple idea -- the two interpenetrating cubes. The steel-and-glass cube, a wintergarden that functions handsomely as a lobby and gathering place for institute staff and visitors, is a clear, sharp note, played fortissimo but not in isolation. The masonry cube, sheathed in textured rose-colored blocks, supplies a strong counterpoint. The contrasting qualities -- cool/warm, transparent/opaque, void/solid -- combine to make a vigorous whole.

Through and through a building in the modernist tradition, the institute headquarters adds a distinctive accent to, but sits comfortably in, a neighborhood of mainly older, mainly brick structures. The masonry part of the new building pays respects to the local tradition, but on its own terms. Its windows, for instance, echo the limestone-framed windows of the Henley Park Hotel across 10th Street, but, framed with white-painted steel sheets set off by deep reveals, they're expressively up-to-date. A modern building that fits, this is a fresh approach to the game of Washington contextualism.

Obviously the building provides welcome new visibility for an institution that, in Crane's words, wants to become "a major focal point for policy debate in the nation's capital." It serves the institute's needs well in other ways too. There's a commodious auditorium on the basement level (designed by Gensler & Associates, also responsible for the warm/sleek office interiors), connected to the wintergarden via a sweeping stairwell (HOK's work).

Beyond facades and lobby, there are few opportunities for architectural expression in a little office building. George created a few extra ones, and took advantage of all of them. The existence of the wintergarden created the need for an interior facade. Reflecting the steel-and-glass grid of the main wintergarden wall, but much weightier in character, this north-facing wall comprises an emphatic sequence of solids and voids, with huge crosses of white-painted drywall framed by columns and transverse sections of the rose-colored blocks. If you are walking by, be sure to go inside -- visits are welcomed -- because this is a strong piece of work.

Not incidentally, the voids in the wall make room for balconies on all five office floors, so that occupants, says the institute's brochure, can better experience "an atmosphere conducive to the free-thinking analysis that has come to be expected of Cato." In another expression of the symbolism of transparency, the architect created an unusually open ground floor -- the institute's boardroom, centrally situated, is sheathed entirely in floor-to-ceiling glass panels.

If the clarity of thought in this project is exceptional, the execution in places leaves something to be desired. The steel-and-glass walls are especially disappointing when seen up close. Though the steel members are necessarily large -- essentially free-standing, these walls had to be built to resist enormous wind loads -- they lack the kind of startling delicacy in the detail that one expects of high-tech connectors and finishes. To the contrary, the surfaces here are lumpy and bumpy, with weld lines showing everywhere.

Still, there's the matter of the parapet. Added to the top of the building at neighborhood insistence, according to project architect Derwin Abston, it's another case of an opportunity seized. A 10-foot-high masonry strip on all sides of the masonry cube, the parapet hides the mechanical equipment on the roof. Nothing new about that. But it is perforated with a band of square openings, their translucent panels illumined from behind. It's a touch that adds a note of mystery to the play of light at dusk.