Since the 1960s, the nation's capital has become increasingly "atriumized." It seems like every other new office building downtown, along with many in the suburbs, contains a grand, interior space with a glass roof and several ficus trees.

Hotels, corporate or institutional headquarters, shopping centers and multiuse projects are beneficiaries of the "atriumizing" trend in real estate development and design. They join many predecessor civic and cultural buildings designed when atria were thought to be appropriate and economically feasible only for prestigious, monumental edifices.

As a building design concept, the atrium is certainly nothing new. Its prototypes are ancient and geographically diverse. Egyptian, Greek, Roman, Islamic and Renaissance architects all made use of this design strategy. "Atrium," a Latin word, brings to mind the villas of Pompeii or the religious and secular buildings of Paris, Rome or Florence with their colonnaded and cobbled courtyards.

Light, ventilation and privacy were major factors in the making of atria in traditional houses found in many cultures, especially in warm, dry climates. Such traditional courtyards provided necessary, additional facades where windows and doors could be placed, but through which people on the street could not observe household activities. The courtyard was a social gathering space as well as passage space, a storage space, and a space for collecting rainwater and growing plants.

New or old, an atrium building is one organized around an intentionally shaped, centralizing, enclosed space that may or may not be roofed. When not open to the sky, an atrium space may have habitable building stories both above and below it. And it doesn't necessarily occupy the geometrical center of a building. Rather, its centrality is perceptual.

Deciding to create an atrium building occurs very early in the design and development process, for an atrium's form and purpose are inextricably linked to a building's basic intent, its massing and the ordering of surrounding spaces, zoning regulations, building codes, and economic feasibility. Today's architects and developers employ the atrium building type for many of the same reasons that motivated their predecessors centuries ago, persuading the public of its timeless amenity.

An atrium constitutes, above all, a grand, memorable focal space or room. It can unify, visually and functionally, a building's interior, especially in large structures composed of large and small rooms. An atrium, being a centralized reference space, helps occupants and visitors maintain their orientation and sense of location when moving through a building.

Atria can serve as primary places of arrival, even when reached after some number of steps taken from entrances on street facades. Like exterior entry courtyards, they allow people to slow down, stop, look around and pick up additional visual cues about where to proceed. Sometimes, one's ultimate destination within a building may be visible from the atrium.

An atrium also can serve as a giant foyer, acting as a central circulation space feeding subsidiary spaces. It can be lined by passageways or corridors at many levels, while grand (or modest) staircases, escalators and elevators ascend and descend. Consolidating horizontal and vertical circulation within and around a single atrium space facilitates movement and reinforces the atrium's symbolic and practical significance.

Atria with glazed roofs or without roofs bring natural daylight into the middles of buildings. As buildings grow in size and thickness, these broad windows to the sky become especially important for the environmental comfort of these who must spend considerable amounts of time working inside . . . and who otherwise might not get a room with a view or outside window.

Some atria admit and control desirable solar radiation in winter to reduce heating as well as lighting costs. However, atria can have negative energy effects. Gaining unwanted heat through a glass roof in the summer demands substantially increased cooling and ventilation capacity, and losing heat in the winter requires additional heating capacity.

Atria past and present can be veritable gardens, interior landscapes furnished with trees, shrubs, vines, ground covers, trellises, fountains and terraces. They may be designed more to be seen than to be occupied or moved through. In some cases, such indoor landscapes may be more attractive than the landscape or streetscape found outside the building.

Because an atrium greatly increases the number and cumulative length of a building's facades, shops and restaurants can occupy not only ground-level spaces facing streets and sidewalks but also spaces fronting on the atrium space.

In urban settings, an atrium may be well-connected at sidewalk level to surrounding street spaces and even provide through-block cirulation from street to street or corner to corner. With free and easy movement of pedestrians between interior and exterior realms marked by clearly visible and accessible entrances, atrium-facing merchants Historically, it's worth noting that, until the 1960s and 1970s, many jurisdictions outlawed multistory atriums. may do as well as their sidewalk-related counterparts.

On the other hand, some atria have proved difficult for commerical retail tenants. In most instances, they suffer from lack of exposure to traffic because the atria they inhabit do not enjoy immediate linkages to the outside world. Without such linkages, even the most beautifully designed atrium space may not draw in sufficient numbers of potential customers.

Some atriums may be within the building mass to emerge partially and show on the facade. Inwardly facing atrium facades often echo exterior facades, sharing the same decorative motifs, the same materials, the same proportioning and the same fenestration.

But sometimes the stylistic character changes from outside to inside, perhaps radically. The exterior facades of some buildings frequently offer no clue at all that there is an atrium. Like coconuts, these buildings reveal nothing of their surprising contents until you get inside.

Having definite length, width and height, and bounded by the enveloping building mass, an atrium exhibits unique geometrical and proportional characteristics, just like any other architectural space. Atria can be high, long, wide or thin. They can be cubic, oblong, triangular, round, hexagonal, octagonal or irregular. The feelings and sensations one experiences within an atrium space are determined by these visual characteristics, as well as by an atrium's overall volume, appointments, light quality and modes of activity.

Historically, it's worth noting that, until the 1960s and 1970s, many jurisdictions outlawed multistory atriums. They were considered serious fire hazards because the chimney effect could spread flames and combustion gases quickly from lower to upper floors via the atrium. But more sophisticated fire- and smoke-suppression technology, especially increased requirements for sprinkler systems throughout high-occupancy structures, has resulted in building codes permitting properly equipped multistory atria.

In subsequent columns about atrium buildings, several questions will recur: What are the economic consequences of developing atrium buildings? Do commercially successful atrium spaces siphon off street activity to the latter's detriment? Why are some atria animated and vibrant while others appear lifeless and austere? Indeed, could there be too many inwardly focused atrium buildings in a city such as Washington, D.C., much of whose existing streetscape fabric is so rich?

NEXT: A sampling of atria in D.C.