Windows, come to think of it, are amazing architectural elements. They're the "eyes" through which people on the inside of buildings frame and see the world outside. They also allow outsiders to glimpse the particularities of a world inside. They play, in other words, a basic social role. By uniting interior and exterior from either side of the opaque divide of a building's wall, they articulate private and public realms of experience.

They're technological feats too. Window openings were few and far between in humankind's earliest buildings -- they weakened simple structures. In northern climates, even after the coming of structural systems strong enough to permit lots of windows, their deployment was discouraged by cold air. The ancients used glass to cover such openings, but also thin sheets of mica or gypsum or oiled vellum. Only gradually did the development of glass-pane manufacturing techniques permit windows of the kind we're familiar with.

And of course, windows can be beautiful, can stir the soul. Pick any building that you love, and chances are its windows in form and arrangement have much to do with why you love it. Medieval builders devised stupendous engineering systems in order to permit the interiors of their churches to be bathed in transforming light through windows of astonishing size, but one does not have to visit a Gothic cathedral to be transfixed by the power of incoming light. A simple shaft of sunlight falling upon an apple and a tabletop in one's own kitchen can do the trick.

"Windows Through Time," an exhibition that opened yesterday at the National Building Museum, focuses on American windows from Colonial times through the 1930s, and in so doing tells us much about ourselves. The advantage of the show, organized by the National Park Service, is the strictness of its approach -- its heroes are the windows themselves, 30 or so in number, arranged ingeniously upon sturdy networks made of metal tubes.

If, in consequence, these windows seem strangely disembodied, it is an effect that heightens one's appreciation of them as objects, the people who made them and the social conditions that shaped their manufacture and placement. Much of the fun of the show is found in the process of mentally putting Humpty together again, a task much aided by strategically placed photographs and texts.

A visitor's way is prepared by an introductory section on window construction and glassmaking technology -- it's brief but intense. The epochal transition from handcrafted to machine manufacturing, for instance, is economically suggested in the juxtaposition of a case showing the beautiful hand tools of the craftsman -- gouges, planes, mallets, hammers, tenon saws, mortising chisels -- with two late-19th-century machines, also beautiful, designed to do some of the same tasks at 10 times the speed.

Equally informative is the glassmaking display: One can immediately see the difference between the panes cut from twirled molten sheets (the curved lines being the telltale sign), those fashioned from blown glass tubes that were sliced and flattened (the resulting panes are speckled and bubbled), and those made by the "float glass process" closer to our own time (they're just about free of distortion).

The oldest piece in the show is a fragment of a leaded casement window from a 17th-century Northeastern house. Even without the help of a photograph to place it in context one would be able to divine much about the culture that produced such an object, a society battened against a harsh (if also a brave) new world, utilizing much from the abandoned Old World in order to make the transition. One could not see as much from such windows as we're now accustomed to seeing, but then, that was the point in such hearth-centered homes.

Of the two greatest changes in the evolution of window styles and technology during the period covered by the show -- the introduction of vertical-sliding sash windows in the 18th century and the advances of plate-glass technology in the 19th and 20th centuries -- only the first gets due emphasis. This is probably because an adequate demonstration of attainments made possible by plate glass would have involved the reconstitution of, say, an entire 19th-century storefront with its unprecedented areas of clear glass.

In any case, the development of sash windows in itself helped to account for a revolution in domestic architecture. Openings became higher, wider, more numerous and, in the first century or so of their use in this hemisphere, more formally arranged. This followed technological changes -- better glass, more efficient construction techniques -- and also reflected an altered notion of the individual's place, or the family's, in relation both to wild nature and to human society. This cogent pattern of fenestration, repeated with subtle variation in country, town and city, speaks at once of social order and self-awareness. It is perhaps accidental but not at all surprising that the widespread use of sash windows coincided with the burst of confidence that led the Colonies to seek independence.

The durable double-hung sash window is with us still, of course. Today's energy-efficient versions, with double or triple glazing, often come supplied with detachable (and technically unnecessary) mullions for the sake of convention and economy. Though it doesn't bring us up to date, this show contains excellent examples of sash windows in Georgian, Federal, Greek Revival, Italianate, Arts and Crafts and other mixed styles. It's an ample demonstration of why this type of window has lasted so long in a period of rapid change -- it is easy to build, easy to use and extremely adaptable to changes in taste and style.

Obviously, sash windows are by no means the whole story. The exhibition contains examples of hollow-core metal and steel industrial windows, and it ends, in a testament to the tides of taste, with the reintroduction of hinged casement windows in period revival and Art Moderne buildings of the 1930s. The irony of such a conclusion is neat, but one leaves the show with a feeling of unfinished business -- the history of windows in the latter half of the 20th century is in itself an astounding one.

Today we can and do build glass houses and glass buildings; we can fashion windows of almost any shape and can puncture the walls with them almost anywhere we please. We're entering a period when the pull of the future again is strong -- anything's possible! And yet the pull of convention remains -- why do just anything? This is in truth another chapter, or even another story altogether. Next time, perhaps. Meanwhile, these old windows in all their authentic, isolated glory will remain on view at the Building Museum, F Street between Fourth and Fifth streets NW, through March 31.

Very much worth seeing too is a new exhibition devoted to the planning and construction of the Blue Ridge Parkway, that exemplary design-with-nature road that bends with the Appalachian Mountains for 470 miles, linking national parks in Virginia and North Carolina. No hurry: This exhibition continues at the museum through May 3. However, aficionados of local architecture will not want to miss "Give Us Your Best," an extensive show of contemporary work by Washington firms that lasts only through Dec. 3.