This column is about an exception to an unhappy rule -- a splendid new residential tower at the intersection of Massachusetts Avenue with H and Fourth streets NW, one of those made-in-Washington triangular sites.

What makes the building splendid is, quite simply, the quality of its architecture. This is a brick edifice that celebrates brick, a triangular structure that dramatizes its shape with subtly undulating walls, a certifiable Washington building that nonetheless breaks the old mold.

What makes the building an exception is the mediocre architectural quality of a trio of nearby residential buildings, also new. Unfortunately, mediocrity is the great character flaw of most of the new residential buildings downtown.

Make no mistake, the fact that these buildings even exist is an economic plus. It used to be, when you saw construction cranes crowding the downtown skyline, you could be sure another office building boomlet was underway. No longer.

Once again, there are cranes all over the place. But today chances are strong that those cranes will be hoisting window units to the upper floors of an apartment building or loft condominium.

This change seems almost a miracle to seasoned observers of downtown's fortunes. Plans to create a truly "living downtown," to quote former mayor Marion Barry's decades-old slogan, have failed time and again.

The missing element has always been the same: People who call downtown home. There was a flurry of residential construction near Pennsylvania Avenue and Seventh Street NW during the late 1980s and early 1990s, most of it subsidized by the federal Pennsylvania Avenue Development Corp. But the resulting Penn Quarter neighborhood remained an isolated enclave.

Until now. Penn Quarter is being joined by high-density residences to the north, mainly in Chinatown and on both sides of Massachusetts Avenue. Besides adding to the city's tax base, the new residents are sure to have a strong impact on the liveliness of downtown streets. This is important. A healthy center is vital to the entire city.

Still, high costs are a cause for concern. Monthly rents of $2,500 are not rare, and condo prices can soar to $1 million or more. This price inflation threatens the stability of low-income neighborhoods in Chinatown and north of Massachusetts Avenue. Such prices also make one wonder about the long-term sustainability of the downtown housing market: The supply of single entrepreneurs, childless two-lawyer families and empty-nester suburbanites cannot, after all, be endless.

So far, however, so good. Except in architecture! In a city known for the quality of its apartment buildings, this is an astonishing oversight.

Most developers of large downtown residential towers promise to provide their customers with exercise rooms, party facilities, rooftop pools and other amenities. But they seem clueless to the fact that, more than anything else, high-quality architecture is the guarantor of long-term viability -- for an individual project, and for a neighborhood.

Of the four new buildings on a four-block stretch of Massachusetts Avenue, for instance, only one meets this crucial quality test. It is the 262-unit condominium tower at 400 Massachusetts Ave. NW, developed by Faison Enterprises and designed by the Washington firm of Philip Esocoff & Associates.

About the three others, the best I can say is that they get passing grades. Meridian at Gallery Place, the largest of the group with 462 units, gets maybe a D. It's a masonry hulk (with nice balconies) that looks as though it got lost on the way to a run-of-the-mill suburban location.

Sovereign Square, a 246-unit apartment building paired with a Hampton Inn Hotel, stretches an entire block on the north side of Massachusetts Avenue between Fifth and Sixth streets NW. It deserves perhaps a solid C. Its most interesting characteristic is the change in style between the drearily conventional hotel and the slightly peppier, more contemporary, more metallic wing housing the apartments.

Massachusetts Court, on the south side of the avenue between Third and Fourth streets NW, merits possibly a B-minus. It also is a two-part building. The avenue-facing segment, containing 297 apartment units, is a red brick structure with identifiable base, middle and top -- an inoffensive take on the pre-World War II Washington standard. The other part, containing 74 loft-style units, appropriately faces Fourth Street with a livelier, more colorful, more open facade.

Esocoff's building, by contrast, is an A. It's a spirited structure, at once seriously idiosyncratic and respectfully playful.

The idiosyncrasies start with the shape. Architects for generations have taken advantage of Washington's many narrow corners, created by all the diagonal boulevards, to make emphatic rounded or pointed forms. Esocoff does the same, with a twist, by curling a slice of masonry away from his rounded corner at the very top. It's like a flag, improbably made of bricks.

Then there are the wavy facades to the north and south. From the inside, they provide residents with long urban views. From the outside, they transform what could have been overlong elevations into elegant, rhythmic facades unlike any others in the city.

The architect points out that the curves are based on the city's basic building technology of concrete structural columns and concrete floor plates. It's not terribly hard, Esocoff says, to curve the ends of the floor slabs and thereby "get a lot of effect for a small additional amount of money." Not terribly hard, perhaps. But not often done.

Esocoff is one of the few modern architects who'll even admit to admiring ornament. (Another is his wife, Amy Weinstein. Not surprisingly, both studied with decoration-conscious Pritzker Prize winner Robert Venturi at the University of Pennsylvania.) At 400 Massachusetts Ave., this admiration is expressed in strong, simple, colorful brick patterns that reinforce the traditional base-middle-top divisions of the facades.

The architect also enhances the liveliness of his facades with calculated shifts in window sizes and patterns, and changes in materials and textures. This is a building that has a certain billboard effect when you are driving by -- but, like most really good buildings, it reveals its qualities much more fully at walking speed.

The same unfortunately cannot be said for most of the large, architecturally lackadaisical new residential buildings downtown. We should do better. And, as 400 Massachusetts incontestably proves, we can.

The 262-unit condominium tower at 400 Massachusetts Ave. NW, designed by the local firm of Philip Esocoff & Associates, uses curved slabs, changes in texture and varying window sizes to create elegant, rhythmic facades.