In the 1950s, an unforgettable TV episode of "Amos 'n Andy" concerned a two-story, suburban-house facade about 6 inches thick. Erected on an attractive subdivision lot as a background set for shooting a movie scene, it looked genuine.

The irrepressible Kingfish, having assumed the role of real estate agent, set about to market this "unique" property to his favorite customer, Andy. The most memorable scene occurred when Andy, accompanied by Kingfish, admiringly approached the house, opened the front door and stepped in. He immediately found himself in the back yard. Meanwhile, Kingfish was extolling the virtues of the property -- how quickly one could reach the garden from the inside, how little effort was needed to keep its compact rooms clean, how wonderful to have so much yard space.

Andy kept walking back and forth through the facade, puzzled that he could get from the back to front yard so quickly, at the same time barely noticing the spaces within. Kingfish kept focusing Andy's attention on the facade and the yards, suggesting that the building's spatial contents were of little consequence.

In retrospect, this bit of comedy seems to be an appropriate commentary on architectural perceptions. Indeed, your visual memory of the environment consists largely of the outer layers of inches-thick facades that you see. But what is exposed to Mother Nature on the outside may reveal little about the exterior wall itself, much less about the building contents.

How are such walls made? Until the 19th century, buildings of consequence always were constructed of solid masonry -- brick, natural stone, cut granite or limestone -- mortared together and finished on the inside with paint, plaster or, budget permitting, marble. In most cases, walls were monolithic and quite thick, being obliged to support their own weight plus the weight of all floors above.

Although apparently strong and permanent, monolithic masonry readily conducts heat and allows water to infiltrate, dampening interior surfaces. Water also penetrates into small cracks caused by seasonal expansion and contraction, gradual shrinkage of mortar and structural movement (primarily foundation settlement). Eventually, through erosion and freezing/thawing cycles, the wall can deteriorate or even collapse as mortar and particles of masonry wash away.

Wood is a much better natural thermal insulator than masonry, and wood structures can absorb slight dimensional changes without coming apart. Nevertheless, wood swells when wet, shrinks as it dries, warps and splits. Susceptible to rot and appetizing to termites, it burns enthusiastically.

Although wood is stronger in tension than masonry, which has almost no tensile strength, masonry is much stronger in compression. Therefore wood, used routinely for houses, barns and other modest buildings, rarely has been used for tall structures. This engineering reality, coupled with wood's rustic associations and vulnerabilities, continues to account for its being judged inappropriate for monumental Western architecture.

During the last hundred years, metallurgy, chemistry and chemical/petroleum engineering gave rise to unprecedented new products for building construction, changing the way buildings are shaped. Structural steel, 20 times stronger than masonry and producible in a variety of shapes from thin sheet to round tubes, permitted open-frame, high-rise building. Steel and aluminum could be rolled or extruded to make lightweight wall panels, studs, window frames and sash.

Evolving synthetics industries invented and manufactured plastic and rubber compounds that could be squeezed out of tubes to fill and seal construction joints. Continuous, amazingly thin sheets of plastic, rubber, aluminum or copper could be fabricated for use as moisture barriers. Mineral and glass industries devised new fibrous insulations in which millions of tiny air pockets, each resisting heat flow, are entrapped.

Clearly, traditional ways of designing exterior walls of buildings became questionable. Architects realized that, in an age of increasing specialization, the facade's many tasks could be performed by separate, specialized products and systems that nevertheless could be combined to make a lightweight, unified wall assembly. They also realized that these new technologies allowed much greater compositional freedom than had been possible when walls could be made only of heavy, compressive masonry.

Consequently, most 20th-century building facades are not monolithic, but are rather sandwiches composed of several distinct layers of materials. These layered sandwiches are placed in or hung on independently structured steel or reinforced concrete skeletons that support all the building's weight (called "dead loads") plus all superimposed loads ("live loads") arising from occupants, furnishings, equipment, wind and earthquakes.

Imagine a cross-section of a generic curtain wall with the outside to your left, the inside to your right.

Exposed directly to the weather on the left is the finish "siding" or "cladding," the outside veneer. It can be brick, metal panels, stucco on lath, terra cotta, pre-cast concrete, cast stone, limestone, granite or marble. Veneers are expected to provide desired exterior color and texture, impermeability to water, resistance to staining or discoloration, durability and ease of maintenance. Usually presumed to last as long as the building, veneers can be less than an inch in thickness. They contribute little to a building's structural stability.

Veneers are backed up by and fastened laterally to a layer of compressed mineral sheathing board, typically 1/2 to 3/4 inches thick, attached to the outside of 3- to 6-inch vertical studs spaced 16 to 24 inches apart. Or concrete block, 4 to 12 inches thick, may be used as a backup wall.

Stud or block backup assemblies, connected to the building structure, provide curtain wall rigidity. Glass fiber insulation, slightly less than the stud thickness, resides between studs. For block backup walls, rigid foam insulation between 1 and 3 inches thick is laminated to one side.

An air space, or "cavity," between the veneer and the sheathing or block backup wall provides an important thermal break. The cavity ventilates the wall, allowing water vapor and condensation trapped within the wall eventually to migrate out via "weep holes," or thin drainage slits through the veneer.

At the right in our generic wall section is the interior finish -- typically 1/2-inch gypsum board ("drywall") -- applied directly to studs or to "furring" strips, 3/4 or 1/2 inches thick, attached to concrete block walls. Drywall's paper surface may be painted, or additional wall finishes -- ceramic tile, wallpaper, fabrics, plastic wall coverings, wood paneling -- can be added.

On the room side of the insulation is a vapor barrier, often aluminum foil or polyethylene. Its mission is to prevent water vapor generated inside the building (e.g., from cooking, bathing, humidification) from penetrating into the exterior wall. During cool weather, this vapor can condense upon reaching a layer in the wall sandwich where the temperature is at or below the dew point. Condensation can saturate insulation, destroying its effectiveness, and ultimately damage other wall materials.

A wall can be as thin as a few inches, or as thick as a foot. Its overall dimension is simply the sum of the dimensions of each layer chosen -- cladding, air space, sheathing and studs (or block wall backup) with insulation and interior finish. In general, the greater the floor-to-ceiling distance the wall must span, the stronger and thicker the wall must be to achieve sufficient lateral stiffness. But the designer also may thicken a wall for other reasons, either aesthetic or technical.

Remember that a building's facade is really three dimensional. Thus, our exploration is not finished. For what happens when the architect cuts or garnishes the facade sandwich?