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Ruins of the floor of a late Roman villa. The floored part is the exedra. The rest of the room disappeared and shows the columns of the hypocaust.

In architecture, an exedra is a semicircular recess, often crowned by a half-dome, which is usually set into a building's facade. The original Greek sense (a seat out of doors) was applied to a room that opened onto a stoa, ringed with curved high-backed stone benches, a suitable place for a philosophical conversation. An exedra may also be expressed by a curved break in a colonnade, perhaps with a semi-circular seat.

The exedra achieved particular popularity in Roman architecture during the Roman Empire. In the 1st century CE, Nero's architects incorporated exedras throughout the planing of his Domus Aurea, enriching the volumes of the party rooms, a part of what made Nero's palace so breathtakingly pretentious to traditional Romans, for no one had ever seen domes and exedras in a dwelling before. An exedra was normally a public feature: when rhetoricians and philosophers disputed in a Roman gymnasium it was in an exedra opening into the peristyle that they gathered. A basilica featured a large exedra at the far end from its entrance, where the magistrates sat in hearing cases.

An exedra adopted by James Cameron for a neoclassical interior space, at the Hermitage

Following precedents from Rome, exedrae continued to be in widespread use architecturally after the fall of Rome. In Byzantine architecture and Romanesque architecture this familiar feature developed into the Apse and is fully treated there. A famous use of the exedra is in Bramante's Belvedere extension of the Vatican palace.

In Muslim architecture, the exedra becomes a mihrab and invariably retains religious associations, wherever it is seen, even on the smallest scale, as a prayer niche.

Both Baroque and Neoclassical architecture used exedras. Baroque architects (for example, Cortona in his Villa Pigneto to enrich the play of light and shade and give rein to expressive volumes, Neoclassical architects to articulate the rhythmic pacing of a wall elevation. A classic example of a Baroque exedra on a (comparatively) reduced scale within its context, is the central niche of the Trevi Fountain (illus. at that entry) in Rome, sheltering a statue of Neptune.

The interior exedra was richly exploited by Scottish neoclassical architect Robert Adam and his followers. During the 18th century an exedra became a popular garden feature or folly, often used as an ornamental curved screening wall to hide another part the garden, examples can be found at Belton House and West Wycombe Park.

Many classicizing bandshells in public parks are exedrae, for the shape, with its half-dome heading, reflects sound forwards. The Hollywood Bowl's shell (illus. at that entry) takes the form of the head of a gargantuan exedra, stripped of classicizing details.

• LacusCurtius website: "Exedra," in Smith's Dictionary of Greek and Roman Antiquities, (1875)