Why do some urban streets, spaces and places become animated, active and memorable, while others seem lifeless, formless or inhospitable . . . no matter what the style or quality of the architecture around them might be like?

When a building in the city is designed and built, it is much more than just another journal entry in historians' accounts of periods and styles, in its architect's list of projects built, or in its owners' ledgers of income and expense. While individual buildings have their own separate identity and internal life, they interact with other buildings and spaces around them to constitute some larger environmental experience.

But city building spans dozens of decades, an inevitably piecemeal process characterized by great diversity in architectural styles, ownership and building intentions. How then can any sort of ultimate coherence result?

Visualize Washington, its different neighborhoods and special precincts. Your recollection may focus sometimes on individual buildings easily remembered and sometimes on the character of a space -- a street, circle or plaza -- contained and shaped by a collection of buildings, no one of which dominates. Occasionally, spaces and buildings together achieve equal presence, as exemplified by the Mall and its monumental edifices.

In commercial areas away from downtown, urban-scaled public space-making, if it occurs at all, is almost always subordinate to the making of free-standing, object buildings -- office blocks, shopping strips, fast-food outlets, gas stations, retail malls -- surrounded by landscapes of parked cars, lawns, trees and shrubs. Here, formed spaces are typically limited to sidewalks between storefronts and car bumpers, entry lobbies and the interior atrium spaces of shopping centers.

The phenomenon of streetscape dominating individual buildings is perhaps clearest in places such as Georgetown, where some combination of location, accessibility, population density, mixed activity and architecture have produced a "there" there.

Most Washingtonians know there is something unique about Georgetown, in addition to its history, that makes it so active, perpetually busy, crowded, traffic-jammed, yet spatially coherent and memorable (not to mention pricey in its real estate values).

Look first at its location. It fronts on the Potomac River and adjoins the central business district to the east. Key Bridge brings Virginians to and from its west end at M Street, which in turn connects to the western terminus of Pennsylvania Avenue. Wisconsin Avenue, beginning at the K Street river edge, bisects Georgetown and heads upland through the affluent Northwest, Bethesda and on to Rockville. P and Q streets serve as additional east-west links to downtown.

Despite the still-incredible and short-sighted omission of Metro subway service to Georgetown, this centrally located part of Washington remains reasonably accessible by car, bus, or on foot. More important, within Georgetown the pedestrian, not the automobile, reigns.

Georgetown's street alignments and widths are based on the original 18th century grid. Building setback lines are at or close to the sidewalk line. Narrow streets, lined snugly by closely spaced or party-wall buildings, offer pedestrians a sense of enclosure and intimacy lacking in most urban streetscapes dedicated primarily to accommodating great volumes of traffic.

The typical three- to five-story height of Georgetown's buildings, usually a bit less than the width of the street space (the distance between facades on opposite sides of the street), further enhances awareness of the street's potential as a public room. Although individual structures are rich in scale-imparting detail -- steps, railings, porches, shutters, patterns of masonry, windows, trim -- that continually arrest the eye, the facades nevertheless work together to form a continuous, comfortably high wall, holding and shaping the street space.

Take note of the multiple roles played by the exterior walls of buildings. From the interior, they are the enclosing facades of individual rooms, but from the exterior they must represent the whole building. In turn, they are somehow obliged to participate in making the See LEWIS, F14, Col. 1 street wall continuum . . . and all while controlling the passage of light, heat, wind and water.

Georgetown's narrow streets and frequently clogged traffic make it easier for pedestrians to take over the streetscape, crossing from one side to another between waiting cars or at cross-walks. Walking on one side, you can readily shout and communicate with someone on the other side. You also can see what's displayed in shop windows across the street, something not possible on most downtown streets.

Georgetown's vitality derives not only from its sidewalks -- they are not overly wide -- but also from the proximity of storefronts to sidewalks and street curbs along Wisconsin and M, the two major shopping streets. These critical dimensions and juxtapositions establish immediate and unavoidable visual contact between pedestrians, and between pedestrians, motorists and merchandise. No deep arcades redundantly line the sidewalk.

What else brings people to Georgetown rather than other places in the city, places with bigger buildings, more parking and wider streets? Besides chic shopping or partaking of the "esprit de carnival" outdoors, there is eating, drinking and socializing. Restaurants from haute cuisine to fast food abound, along with numerous bars and taverns open after midnight.

And many, many people live in Georgetown. Most live on quaint residential streets, but lots live in apartments above stores and in more recently developed mixed-use structures such as Georgetown Park. There are also hotels, offices and office buildings, schools and a major university.

Thus Georgetown is a city in microcosm, a fairly dense urban village with reasons for people being there all the time, not just Monday through Friday, lunchtime, weekends, or nights. The intense intermixing of activities and land uses, coupled with the physical form of its streetscape environment and historical, architectural charm, give Georgetown its unusual vitality, notwithstanding horrendous traffic and parking problems.

Georgetown is, of course, exceptional. But it illustrates well the importance of reinforcing the streetscape and the pedestrian pathways along street edges with varied activities (including dwellings) housed in well-proportioned buildings. In a city, such buildings must connect directly and actively with the sidewalk/street environment to serve both tenants and passers-by. You need only to look again at L'Enfant Plaza or Rosslyn to realize that these "designed" environments lack something.

It is, of course, easier to describe or imagine an idealized streetscape than to build one. Coping with constraints imposed by location, market forces, zoning, government regulations, safety requirements, traffic, parking, land values and financing makes the task very difficult. However, there is no lack of real-life models here, in other American cities and in urban settlements around the world. They have much to teach us.

NEXT: Sidewalks and the bottoms of building.