The atrium, an ancient architectural idea, often seems remarkably theatrical and futuristic in its modern manifestations. How many science-fiction films have you seen with scenes shot in spaces resembling the interiors of a Regency Hyatt hotel or the National Gallery's East Building?

What makes these spaces so fantasy-like, tantalizing to be in and suggestive of places in some other world or century?

Size is one factor. Atriums may be tall and wide enough to contain easily a multistory building or an interplanetary spaceship. Or they may be long enough to accommodate an ocean liner. In such spaces, your senses seek a scale of measurement and dimensional reference while your imagination speculates.

Atrium construction technology further contributes to this potentially mind-expanding experience. Atrium roofs and exterior walls are often fabricated with an elaborate, three-dimensional, exposed framework of tubular steel spanning great distances.

These transparent structures -- space frames, trusses, girders and beams -- typically have complex connections visible from below that serve as high-tech ornament. Sheathed with glass, such structural forms create dramatic silhouettes against bright backgrounds of diffuse light.

Seeing multiple floor levels from atrium top to bottom may further enhance speculation. Might it be a cross-section of some piece of a future Orwellian city where inhabitants, like the environment, are carefully controlled and tempered?

Indeed, the fantasy-inducing potential of such atriums can lead to theatrical behavior. At the corner of Connecticut Avenue and Van Ness Street is the Intelsat building, designed by John Andrews and composed of a series of cosmic-looking, ingeniously linked office "pods" and atrium spaces crisscrossed by overhead pedestrian bridges.

It's reported that an employe, obviously unable to resist Intelsat's galactic suggestiveness, dressed up as "Star Wars" Darth Vader and paraded through Intelsat's atrium.

But there is more to atrium allure than futuristic technology or fantasy. An atrium can become a kind of festive, real-world theater, a place where people can come to see and be seen, to act, to pose and to consume. Shopping center designers and developers have long capitalized on this phenomenon, recognizing that people are attracted by festivity as well as by the pragmatic conveniences of parking, varietal and concentrated shopping and weather protection.

Festive theatricality is well-represented by the Pavilion in the Old Post Office Building at Pennsylvania Avenue and 12th Street NW, which also houses the National Endowment for the Arts and Humanities on its upper floors.

Completed in 1899, this massive Romanesque revival structure, designed by W. J. Edbrooke, was saved from demolition in the 1970s through the efforts of many thoughtful Washingtonians.

Restored by the General Services Administration, with architectural assistance from Arthur Cotton Moore Associates, it contains a 10-story-high roofed atrium about 190 feet long and 100 feet wide. Like many of its contemporary successors, its roof is framed with exposed steel trusses.

Another set of trusses, supporting only themselves, spans the atrium just above the main floor levels occupied by the Pavilion. They create an implicit, structurally outlined tent canopy, as if a circus might occur below.

As a result, there are really two zones, one below the other, within the single atrium space. The Pavilion, developed by the Evans Co. and designed by Benjamin Thompson & Associates, occupies the lower zone's three levels. It contains cafe's, restaurants, shops, stage and seating areas.

Entering the Old Post Office Building's atrium the first time is a great surprise. You arrive at the sides or corners, not on the atrium's axis. Your view and movement are arrested momentarily by the incredible scene, and one of your first responses will be looking up to measure the awesome space conceived by architect Edbrooke. But soon you look down again.

You quickly become oriented, realizing that the main level connects directly to the lower level via a broad staircase located astride the building's principal axis. Most important, the north half of the main atrium floor is carved away so that there is a visual merging of the two levels.

A balcony level sits atop the roofs of the main level's retail shops at the south end. This third restaurant level occupies less than one fourth of the atrium area. Thus, from many positions in the atrium, one can simultaneously see the bottom (stage) level, plus the balcony and main levels, stepping down to them from south to north.

The Old Post Office atrium space has an organization and an order that are quickly deciphered and understood, aided by the interior landmarks of tower and grand stairways.

The Pavilion bristles with hundreds of light fixtures glowing yellow and white, colorful banners, signs, potted plants, brass hardware and cafe' furniture. Umbrellas and rotating ceiling fans describe circles overhead. Blond oak and Victorian green trim wrap horizontally around wall surfaces, while gray and salmon-colored ceramic tiles establish patterns on the floor.

Above the Pavilion proper, with its truss canopy, is the upper zone of the Old Post Office atrium space. Buff and cream-colored, its soaring interior facades are formed by pilastered piers surmounted by small and large semicircular arches. Wide corridor galleries are open to the atrium between piers, and a series of vertical white banners hangs in a single plane directly below the ridge of the roof.

Yet, the higher reaches of the atrium's upper zone almost disappear by contrast. This part of the space seems quiet and subdued, a separate environment clearly related to the offices more than to the Pavilion. In fact, to see its glazed roof structure from below, you must tilt your head back quite far. Otherwise, you could spend hours in the Pavilion and never really see the top of the atrium.

The Old Post Office's new life now seems predestined. How could it not work -- a giant room in a unique building, in a great location; its bottom filled with eating and shopping attractions; its perimeter accommodating federal offices. For that matter, the latter could just as easily be hotel rooms, because the success of the bottom depends very little on what occupies the upper levels.

There seems to be a perfect fit between the building's immense form and the collective, celebratory nature of the carnival-like activities that occupy it day and night, seven days a week. If the same commercial uses were located in the bottom floors of a conventional building, could such a place ever be comparable either economically or experientially?

Nearby National Place, developed by Quadrangle and designed by Frank Schlesinger and Mitchell/Giurgola, contains a lively three-level atrium within the Northeast quadrant of the block at 13th and F streets NW.

The Shops at National Place, a Rouse Co. undertaking, surround the atrium and line the interior pedestrian street leading diagonally to the Marriott Hotel and Pennsylvania Avenue on the south side of the block.

Like the Pavilion, its combination of stores and restaurants, its accessibility and its festive decor attract visitors and shoppers in droves. But unlike the Pavilion, The Shops' atrium is imbedded within a modern office building that actually rises above it. Natural light enters on the atrium's south side and not through its roof.

The Shops at National Place and the Pavilion are the downtown counterparts of suburban shopping malls. They add much to the life of the city. But how many can the city ultimately absorb? Will everyone soon be shopping inside atriums, rather than along sidewalks and streets?

Even festive atriums survive and thrive only as long as commercial tenants attract enough customers to buy their merchandise. Festivity and fantasy alone won't pay the rents.

Next: Decorous atriums