Already, it passes the taxi driver test.

Even though the building has been occupied for just a month or so, hackers know precisely what it looks like as soon as you give the address -- 500 New Jersey Ave. NW.

Or, to be precise, that has been my experience in two recent visits. "Oh yes, that's that beautiful new building they've put up over there," said one driver. On a second trip, another said pretty much the same thing.

The reason becomes apparent when you make the easy turn from Massachusetts Avenue south onto New Jersey: A memorable vista fills the front windshield. In the distance, the Capitol dome. In the right foreground, an astonishing new landmark.

Commissioned by the Chicago-based National Realtors Association to become its Washington headquarters, the new building is notable in a number of ways. It is, for instance, tall and thin. Remarkably thin.

Standing on a narrow triangular lot formed by New Jersey Avenue as it slices through First Street between E and F streets, the building tapers from 60 feet wide on the south to about 10 feet on the north. These dimensions accentuate the building's 130-foot height. In comparison with blocky buildings of similar height nearby, this one seems to soar.

Graham Gund Architects of Cambridge, Mass., designer of the building in collaboration with SMB Architects of Washington, emphasized these proportions by placing a tapering steel structure at that narrow northern prow. It is a surprising and effective urban exclamation point.

In fact, just about every move the architects made in designing this building reinforces the impression of sleek, self-confident elegance.

Measuring about 170 feet south to north and facing New Jersey Avenue and First Street, respectively, the building's long, all-glass facades curve outward ever so slightly -- a subtle move that yields dramatic, aerodynamic results.

Laura Cabo, a design principal in the Gund firm, explains that these facades, projecting a few feet beyond the building's high ground-level floor, are in accord with a Washington building regulation originally conceived to encourage the construction of protruding bay windows. "We treated the whole side as a bay," she says. This is at once an innovative and highly successful interpretation of an old rule.

Furthermore, the glass on these two impressive "bays" is tinted a striking azure hue, giving the building a jewel-like finish that is pleasing even on rainy days. Under clear blue skies, the reflective effects can be mesmerizing.

Reflective glass facades became a cliche in the 1950s and 1960s, of course, but this elegant design is a refreshing revival. The building is, in effect, a convincing work of abstract sculpture. This is entirely fitting, given its position on a highly visible, triangular island with a tremendous view of the Capitol.

"We wanted this to be a celebration of a Washington crossroads," says Graham Gund, and that it is. Though views of the Capitol from the upper floors and roof deck are predictably spectacular, the crossroads view in the opposite direction is even more dynamic.

Through the narrow aperture in the north, framed by the crisscrossing members of that steel exclamation point, a fascinating slice of the "other" Washington reveals itself. In the foreground, of course, you see a typical D.C. pattern as the diagonal avenue cuts sharply through the north-south street.

Because of its slim dimensions and glass facades, the building is filled with natural light. In addition to being pleasant and healthful, this saves money and energy because the electrical lighting system was designed to adjust automatically to levels of light from outdoors. The exterior glass, not coincidentally, was treated to reduce heat gain.

These are but two of the building's many "green" features. Perhaps the most innovative, if invisible, piece of sustainable architecture is the concrete cistern buried beneath the building. It collects rainwater from the roof for use in irrigating the street trees and other plants around the building.

Iris Amdur, a Washington consultant for sustainable design, is confident that the building will become the first new office building in Washington to be certified by the U. S. Green Building Council. This is an achievement well worth emulating. Certification comes only after a building conforms to many items on a long and demanding list of qualities signifying "Leadership in Energy and Environmental Design."

The Gund design was chosen in a competition sponsored by the Realtors Association several years ago. When it was first unveiled, the design drew a lot of vehement, articulate and, to my mind, wrong-headed criticism.

The opposition was partly stimulated by affection for a traditional design proposed by Jay Hellman, a previous owner of the site. And partly, it seems, it was based on a dogmatic dislike of modern architecture.

Ronald Lee Fleming, president of the Townscape Institute of Cambridge, Mass., wrote that the Gund building was unsuitable "from the point of view of contextual design and sustainability." The traditional design, he continued, was "clearly more historically contextual, sensitive to its culturally significant location and appropriate from an urban planning standpoint."

It seems clear to me, however, that the finished building supports an opposite argument. The Realtors' building is, demonstrably, environmentally responsive. Its design is "historically contextual" in that it celebrates its own historical period instead of pining after some 19th-century style. With great finesse, the design takes advantage of its important location. And, unmistakably, the building contributes significantly to the vitality of its immediate neighborhood.

It is wonderful to look at and pleasant to be near. The setting, designed with admirable restraint by the Washington landscape architecture firm of Oehme, van Sweden & Associates, invites passersby to pause for a while. The space will be further enlivened when a restaurant and a delicatessen open on the ground floor next year.

The Blue Sliver is, in other words, a fine addition to Washington's limited supply of first-rate modern architecture. In a half-century or so, when someone tries to tear the building down, preservationists rightly will say no.

The Realtors Association HQ is a fine addition to a small supply of first-rate modern architecture. The National Realtors Association's Washington headquarters, clockwise from above: a view of the Capitol from a corner office; the impossibly thin building slices through its space; a tower rises from the north side; its dimensions accentuate the building's 130-foot height.