A pediment is an architectural feature which consists of a triangular ornament placed on top of a structure or feature such as a gable. The face of the pediment, known as the tympanum, is often heavily decorated, while the pediment itself is enclosed along the sides with raking cornices, and along the bottom with a horizontal cornice. There are a number of variations on the pediment, and this decorative piece of architecture shows up in a number of different architectural styles; almost any style which features ornamentation of the outside of a structure includes pediments.
The pediment appears to have originated in Ancient Greece, where it was used as the crowning glory on Greek temples such as the Parthenon in Athens. Pediments were classically supported by columns. Some Roman architecture also included pediments, influenced by interactions with Greek culture, and the pediment never truly went out of style, although its popularity certainly waxed and waned.
In addition to the basic triangular shape of the traditional pediment, it's also possible to see a variation known as a segmental pediment, in which the raking cornices create an arch or half circle, rather than a triangle. Either style can be broken, meaning that the tops of the raking cornices don't meet, allowing sculptural elements to protrude beyond the line of the cornices, or open, in which the horizontal cornice is absent.
Another variation is the scroll or swan necked pediment, in which the raking cornices are S-shaped, and said to be reminiscent of the necks of swans. In all cases, the tympanum can be decorated with carved ornamental features, along with decorative painted designs. More modern pediments might utilize mosaic or plaster decorative features set into the tympanum. In some forms of architecture, the ornamentation may be accomplished with multicolored stone or wood which creates a vivid design which can be seen from a distance. More classical versions stick to carvings in a single color of stone, or to decorative wooden scrollwork in a single contrasting color..
In classical architecture, symmetry and clean lines were highly valued. Pediments were centrally located or organized in a symmetrical way, and they blended harmoniously with the structure as a whole. In modern architecture, pediments are sometimes offset for more visual interest, although this can conflict with the classical look of pediments if it is not done with care. Pediments can also be seen decorating window gables, front porches, and similar features of a structure.