A facade, says Webster's dictionary, is "the front of a building." But in Webster's inimitable and succinct way, its second definition states that a facade is "the front part of anything -- often used figuratively with implications of an imposing appearance concealing something inferior."

How often the reverse is true when an otherwise respectable building is compromised by an inferior facade.

Most architects would admit that it is often easier to develop a good set of plans for a project than to create a good set of facades. Deciding how to mass and site buildings, organize them spatially and modulate their structural systems can be less challenging than composing their elevations.

Designing building facades nevertheless can be the most exciting piece of the architectural action (for many office buildings, it's the only action). The enclosing planes of buildings offer the greatest aesthetic opportunities for compositional exploration and expression to which the public has direct visual access. Exterior walls of buildings come closest to being like a painter's canvas stretched over a frame, awaiting the artist's hand. Facades, not floor plans or even massing, are where most designers imprint their signatures.

However, building facades hold other meanings for those who create and use buildings. To a developer, the exterior walls of buildings represent a substantial part of the cost of construction. For example, in office building construction, the exterior skin can cost from $20 to $40 a square foot (installed) of skin area. And that's just the basic curtain wall, exclusive of insulation, structural supports and interior finishes. In a downtown Washington project, the complete exterior-wall system may be as much as 20 percent of the total construction budget.

Naturally, owners and architects expect exterior walls to keep out rain, wind, dirt, insects, unwanted sunlight and intruders. Thermal comfort must be ensured by wall insulation that resists the transfer of heat between inside and outside. Vapor barriers must stop the movement of moisture. Facades should resist the transmission of outside ambient noise (from jet aircraft, autos, sirens and unruly mobs) into the interior, unless a window is opened for ventilation.

Building occupants like windows in their exterior walls. Windows can ventilate and admit natural daylight which, if properly controlled, can lower electric lighting costs. Windows admitting winter sunlight can conserve energy and reduce heating costs, something appreciated by landlords and tenants.

Windows offer views to the outside world, allowing occupants to check the weather, peek in on neighbors, observe changing landscapes, or generally watch whatever goes by. And windows, with the light they bring, make us more conscious of the wall itself -- its dimensions, perhaps its materials -- and the interior space bounded and shaped by the wall.

Facades also are obligated to provide security and safety. They must be structurally capable of supporting their own weight while resisting the lateral surface pressure and forces generated by wind or earthquakes. They should be fire-resistant as well. Except for private homes, low-density housing and small buildings of limited or no public assembly, most building codes require that exterior walls of buildings be assemblies of noncombustible materials -- glass, metal, masonry, concrete, tile or stucco.

Finally, facades must be buildable. Contractors must have access to the materials, labor and equipment needed to construct them. The more exotic the facade design and facade materials, the more costly will be erection, particularly if it requires highly specialized craftsmen, installers, scaffolding or cranes.

Perhaps you take most of this for granted. But architects cannot. For even with the incredible functional, technical and economic demands placed on the exterior walls of buildings, designing facades remains an art more than a science. If one could computerize all of the performance requirements summarized earlier, there still would be an infinite number of possible design solutions -- distinctly different facade compositions -- that could satisfy presumably objective equations.

Consider the many aesthetic variables that the architect and client must ponder:

*What overall imagery should be conveyed by the combination of massing and facade design decisions affecting the building's character -- its stylistic and cultural associations? Facades can be high- or low-tech, playful, witty, sober, sedate, inscrutable, chaotic, orderly, awesome, bizarre, historically allusive, story-telling or any combination of these.

*To achieve desired imagery, what should be the dominant and secondary patterns of windows, doors and other openings that affect solid-to-void relationships; materials and joints between materials; and colors and textures?

*How many materials, and what types, should be used, putting aside purely technical and economic considerations?

*Which window shapes, sizes, proportions, and types (e.g., single-pane, sliding, casement, metal, wood) should be used? Windows can be oblong (vertically or horizontally), square, round, half-round, oval, triangular or diamond-shaped, and can be subdivided, in turn, into smaller units.

*How can facades be enriched three-dimensionally by changes of plane (e.g., between wall and window planes); creation of multiple layers of walls, colonades or screens; wall thickness (thick, thin or even diaphanous); addition of subordinate volumes (bays, towers, balconies, canopies, overhangs); or penetrations by other elements?

*What decorative and ornamental elements should be applied -- facias, friezes, bases, beltcourses, window and door trim, mullion trim, columns and pilasters, spandrels, railings, medallions -- and how? These decisions are obviously part of overall pattern-making and image-making decisions.

In grappling with these questions, architects operate simultaneously in objective and subjective realms, relying on rational thought Facades also can express a building's use and telegraph to the outside world the spaces and structure behind, much like a skin revealing the organs and skeleton of the architectural body. Or they can mask what is contained within . . . and intuition, on analysis as well as feeling. Their "eye," as much as their intellect, makes purely visual judgments related to scale, rhythm and repetition, symmetry and asymmetry, opacity and porosity, among many other attributes. And buildings don't have to be just one thing or possess only one set of attributes.

Facades are conceived as whole, three-dimensional things composed of smaller parts that, in themselves, may constitute whole subfacades. Thus facades may acquire central and flanking "pavilions," attic-like tops, middles and base-like bottoms through changes in wall plane and massing, or by manipulation of surface patterns. Subfacades with their own self-defined orders can be organized around major entrances or other significant points within a larger facade order.

A piece of a facade's backside -- let's say a section of wall with three windows in it -- can serve simultaneously as the interior facade of a room. Those three windows should not only enhance and contribute to the architectural form of the room, they also might be positioned on the exterior facade in such a way as to accentuate a doorway below or participate in a sequence of windows forming a distinct pattern seen from a great distance.

Facades also can express a building's use and telegraph to the outside world the spaces and structure behind, much like a skin revealing the organs and skeleton of the architectural body. Or they can mask what is contained within, instead deriving compositional cues from other sources -- the surrounding context, distant architectural precedents or the designer's whim.

Building imagery, determined essentially by facade composition and massing, is what we most readily recall about architecture. But we still only just are scratching its surface.