Because an entry reduces the house's architectural elements down to the human scale, you can learn a lot from focusing your attention on transoms, sidelights, pilasters, and panels.

The following primer, featuring doors from Charleston, will help you identify doors from four distinct American architectural periods.

Georgian (1700 to 1780)
"When the Renaissance began and architects, most notably Palladio, began imitating Roman forms, the only things visible were the remains of monumental buildings, triumphal arches, and coliseums," Martz says. From the classical revival of the Renaissance and Enlightenment emerged Georgian architecture, exemplified by Georgian doors, which, according to Martz, have a "solid, stocky, dignified pageantry that is unique to Roman public architecture."

Georgian entries, most of them boldly confronting the elements without the cover of a portico, follow the style's overall insistence on symmetry, balanced proportion, and classical detailing. Typically, a six-paneled front door stands between a pair of columns or pilasters supporting a decorative entablature. A row of rectangular panes of glass, or lights, runs atop the door, either built into it or above it in the transom. Landmark designs after about 1750, such as the example shown above, feature a semicircular window above the door (though the fanlights are much more common in Federal doorways). Above the Georgian door's entablature often rests a triangular or curved pediment.

Federal (1780-1820)
The Federal style (also known as Adam style) borrows from Roman domestic, rather than monumental, architecture, specifically as a result of the discovery of Pompeii and Herculaneum in the late 1700s. "The residential architecture was delicate and fussy, much more ephemeral," says Martz. "Instead of good sturdy columns holding up a good sturdy entablature, you now have long, skinny attenuated ribbons and bands, swags, and garlands."

In our example, the layout is classic Federal: semielliptical fanlight, delicate tracery found in the fanlight and sidelights, and vertical and horizontal supporting members that emphasize refinement rather than power. Like the Georgian door, the Federal door usually has six raised panels. The heavy beading and intersecting square blocks of the trim, however, are Greek Revival features -- "a little hint of what's coming," explains Martz.

Greek Revival (1825-1860)
While Federal and Georgian styles derived from the Palladian vision of ancient Roman design, Greek Revival architecture turned to ancient Greece for its proportions and ornamentation. Greek Revival doors were recessed, as opposed to the Georgian doors that projected from the house, and the Federal fanlight details disappeared.

Another obvious difference: Many Greek Revival doors are sheltered by porticos, which in Southern locales provide welcome shade and protection from storms. The doorways usually feature some type of columns or pilasters, often copied from ancient Greek structures. Also common is the tripartite arrangement of rectangular transom and two vertical sidelights, which echoes the simple post-and-lintel technique central to Greek construction. Though often spare, Greek Revival doors are not without ornamentation, showing off Greek-key fretwork, acanthus leaves, and palmettes. "A lot of these Greek Revival doors are right out of pattern books," Martz explains, referring to the early sets of home building plans that helped the Greek Revival style spread like wildfire.

Italianate (1840-1885)
A loose and expressive romanticism reveals itself in the flourishes and exaggerated proportions on Italianate doors. For instance, the door in this example is dominated by striking, almost abstract blocks interrupting the curving architrave, as well as oversize dentils in the cornice box.

Many Italianate doors are arched, a defining symbol of Roman architecture. Instead of six panels, Italianate doors often have four, with elaborate moldings, such as the egg-and-dart pattern in this example.