Pediments are a form of gable in classical architecture, usually of a triangular shape.  Pediments are placed above the horizontal structure of the cornice (an elaborated lintel), or entablature if supported by columns. In ancient architecture, a wide and low triangular pediment (the side angles 12.5° to 16°) typically formed the top element of the portico of a Greek temple, a style continued in Roman temples. But large pediments were rare on other types of building before Renaissance architecture.  For symmetric designs, it provides a center point and is often used to add grandness to entrances.

The cornice continues round the top of the pediment, as well as below it; the rising sides are often called the "raking cornice".  The tympanum is the triangular area within the pediment, which is often decorated with a pedimental sculpture which may be freestanding or a relief sculpture.  The tympanum may hold an inscription, or in modern times, a clock face.

The main variant shapes are the "segmental", "curved" or "arch" pediment, where the straight line triangle of the cornice is replaced by a curve making a segment of a circle, the broken pediment where the cornice has a gap at the apex, and the open pediment, with a gap in the cornice along the base. Both triangular and segmental pediments can have "broken" and "open" forms. 

Pediments are found in ancient Greek architecture as early as 580 BC, in the archaic Temple of Artemis, Corfu, which was probably one of the first.  Pediments return in Renaissance architecture and are then much used in later styles such as Baroque, Neoclassical and Beaux-Arts architecture, which favoured the segmental variant.

History

Classical
The pediment is found in classical Greek temples, Etruscan, Roman, Renaissance, Baroque, Rococo, Neoclassical and Beaux-Arts architecture.  Greek temples, normally rectangular in plan, generally had a pediment at each end, but Roman temples, and subsequent revivals, often had only one, in both cases across the whole width of the main front or facade. The rear of the typical Roman temple was a blank wall, usually without columns, but often a full pediment above.  This effectively divorced the pediment from the columns beneath it in the original temple front ensemble, and thereafter it was no longer considered necessary for a pediment to be above columns. 

The most famous example of the Greek scheme is the Parthenon, with two tympanums filled with large groups of sculpted figures.  An extreme but very influential example of the Roman style is the Pantheon, Rome, where a portico with pediment fronts a circular temple.

In ancient Rome, the Renaissance, and later architectural revivals, small pediments are a non-structural element over windows, doors and aediculae, protecting windows and openings from rain, as well as being decorative.  From the 5th century pediments also might appear on tombs and later non-architectural objects such as sarcophagi.

In the Hellenistic period pediments became used for a wider range of buildings, and treated much more freely, especially outside Greece itself. Broken and open pediments are used in a way that is often described as "baroque".  The large 2nd-century Market Gate of Miletus, now reconstructed in the Pergamon Museum in Berlin, has a pediment that retreats in the centre, so appears both broken and open, a feature also seen at the Al-Khazneh (so-called "Treasury") tomb at Petra in modern Jordan.  The broken pediments on each of the four sides of the Arch of Septimius Severus at Leptis Magna in Libya are very small elements, raking at an extremely steep angle, but not extending beyond the entablature for the columns below. There are two faces to each pediment, both carved, with one lying parallel to the wall of the monument, and the other at right angles to that.

The Arch of Augustus in Rimini, Italy (27 BC), an early imperial monument, suggests that at this stage provincial Roman architects were not well practiced in the classical vocabulary; the base of the pediment ends close to, but not over, the capitals of the columns. Here the whole temple front is decoration applied to a very solid wall, but the lack of respect for the conventions of Greek trabeated architecture remains rather disconcerting.

Medieval
In Carolingian and Romanesque architecture pediments tended towards the equilateral triangle, and the enclosing cornice has little emphasis; they are often merely gable ends with some ornament. In Gothic architecture pediments with a much more acute angle at the top were used, especially over doorways and windows, but while the rising sides of the cornice is elaborate, the horizontal bottom element was typically not very distinct. Often there is a pointed arch underneath, and no bottom element at all. "Pediment" is typically not used for these; they are often called a "canopy".  From the Renaissance onwards, some pediments no longer fitted the steeply pitched roofs and became freestanding, sometimes sloping in the opposite direction to the roof behind.

Renaissance and Baroque 

When classical-style low triangular pediments returned in Italian Renaissance architecture, they were initially mostly used to top a relatively flat facade, with engaged elements rather than freestanding porticos supported by columns. Leon Battista Alberti used them in this way in his churches: the Tempio Malatestiano (1450s, incomplete), Santa Maria Novella (to 1470), San Sebastiano in Mantua (unfinished by the 1470s), Sant'Andrea, Mantua (begun 1472), and Pienza Cathedral c. 1460), where the design was probably his. Here the cornice comes out and then retreats back, forming the top of pilasters with no capitals, a very unclassical note, which was to become much used. 

In most of these Alberti followed classical precedent by having the pediment occupy the whole width of the facade, or at least that part that projects outwards.  Santa Maria Novella and Sant'Agostino, Rome (1483, by Giacomo di Pietrasanta, perhaps designed by Alberti) were early examples of what was to become a very common scheme, where the pediment at the top of the facade was much less wide, forming a third zone above a middle zone that transitioned the width from that of the bottom.  The giant curving volute or scroll used at the sides of the middle zone at Sant'Agostino was to be a very common feature over the next two centuries.  As in Gothic architecture, this often reflected the shapes of the roofs behind, where the nave was higher than the side-aisles. 

Sant'Agostino also has a low, squashed down pediment at the top of the full-width section. This theme was developed by Andrea Palladio in the next century. The main facade of his San Giorgio Maggiore in Venice (begun 1566) has "two interpenetrating temple fronts", a wider one being overlaid with a narrower and higher one, respectively following the roof lines of the aisles and nave.  Several of Palladio's villas also introduced the pediment to country house architecture, which was to be become extremely common in English Palladian architecture.  In cities, Palladio reserved the temple front for churches, but in the Baroque, and especially outside Italy, this distinction was abandoned.

In St Peter's Basilica there is a conventional pediment over the main entrance, but the complicated facade stretches beyond it to both sides and above, and though large in absolute terms it makes a relatively small impression. Many later buildings used a temple front with pediment as a highlight of a much wider building. The St Peter's facade also has many small pedimented windows and aedicular niches, using a mixture of segmental, broken, and open pediments.

Variations using multiple pediments became very popular in Baroque architecture, and the central vertical line of church facades often ascended through several pediments of different sizes and shapes, in Rome five at the Church of the Gesù (Giacomo della Porta 1584) and six at Santi Vincenzo e Anastasio a Trevi (Martino Longhi the Younger, 1646), the top three folding into each other, using the same base line.  This facade has been described as "a veritable symphony in repetitious pedimentry, bringing together a superimposed array of broken pediments, open pediments and arched pediments".  The Gesù is the home church of the Jesuit order, who favoured this style, which was first seen in many cities around Europe in a new main Jesuit church.

Variants

A variant is the "segmental" or "arch" pediment, where the normal angular slopes of the cornice are replaced by one in the form of a segment of a circle, in the manner of a depressed arch. Both traditional and segmental pediments have "broken" and "open" forms. In the broken pediment the raking cornice is left open at the apex. The open pediment is open along the base, with a gap in the cornice for part or all of the space under the pediment. 

All these forms were used in Hellenistic architecture, especially in Alexandria and the Middle East.  The so-called "Treasury" or Al-Khazneh, a 1st-century rock-cut tomb in Petra, Jordan, is a famously extreme example, with not merely the pediment, but the whole entablature, very "broken" and retreating into the cliff face.

They were adopted in Mannerist architecture, and applied to furniture designed by Thomas Chippendale. The terms "open pediment" and "broken pediment" are often used interchangeably. Another variant is the swan's neck pediment, a broken pediment with two "S"-shaped profiles resembling a swan's neck, typically volutes; this is mostly found in furniture rather than buildings. Non-triangular variations of pediments are often found over doors, windows, and porches.

Gallery

See also 
Pedimental sculptures in Canada
Pedimental sculptures in the United States

Notes

References
 
 Furman, Adam Nathaniel , "Seven broken pediments", 14 July 2014, The RIBA Journal blog 
Lawrence, A. W., Greek Architecture, 1957, Penguin, Pelican history of art
 .
Summerson, John, The Classical Language of Architecture, 1980 edition, Thames and Hudson World of Art series, 
Yarwood, Doreen, The Architecture of Europe, 1987 (first edn. 1974), Spring Books,   

Architectural elements
Columns and entablature