A Diocletian window — also called a thermal window — is a semicircular window divided by two vertical mullions into three lights: a wider central light flanked by two narrower side lights. The design takes its name from the Baths (*thermae*) of the Emperor Diocletian in Rome (302 AD), where it was used to light large halls from high up.
Shape
A typical Diocletian window consists of:
- A semicircular top — the arch describes an exact half-circle
- Two vertical mullions — supporting the arch and dividing the opening into three lights
- A wider central light — often twice as wide as the side lights
- A flat sill — closing the semicircle at the bottom
Sometimes the window contains no further glazing bars; in other cases it is filled with wrought-iron tracery, leaded glass or a grid of square panes.
Application
The Diocletian window was rediscovered after antiquity and used in several periods:
- Roman Imperial era — the Baths of Diocletian, Caracalla and Constantine
- Italian Renaissance — Palladio used it in his villas and churches; his *Quattro Libri* (1570) made it a stock feature of the classical repertoire
- Dutch Classicism (17th century) — churches, town halls and country houses received thermal windows in pediments, gables or domed drums
- Neoclassicism (18th–19th century) — return in English country houses (Inigo Jones, Robert Adam), French town houses and Dutch villas
- Late 20th century — Postmodernism used the window as a quotation of classical architecture
Function
The Diocletian window has both aesthetic and practical advantages:
- Daylight high up in the space — ideal for large halls where ground-level windows are not desired (churches, ballrooms, theatres)
- Stately appearance — the semicircle directly evokes classical architecture
- Structurally efficient — the arch can span a wide opening without a heavy lintel
Not to be confused with the Serliana
Don’t confuse it with the Serliana (or Palladian) window: that has a semicircular central arch above two rectangular side lights with their own flat tops. The Diocletian window has a single continuous semicircle across the full width.
Related terms
- Tympanum
- Lunette
- Classicism
- Palladio
- Band window
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