What Is a Diocletian Window?

Ancient Roman Influence on Renaissance Architect Palladio

Restored Diocletian Window at the Baths of Diocletian, Thermae Diocletiani, Early Fourth Century AD
Restored Diocletian Window at the Baths of Diocletian, Thermae Diocletiani, Early Fourth Century AD. Photo by Franco Origlia/Getty Images Entertainment Collection/Getty Images

A Diocletian window is a large three-part window with the tops of each window forming a semi-circular geometric arc. Similar to a Palladian window, the center section is larger than the two side sections, but visually the windows appear set within a Roman arch.

More Definitions:

The Dictionary of Architecture and Construction combines the Palladian and Diocletian windows together under the term Venetian window, with this general definition:

"A window of large size, characteristic of neoclassic styles, divided by columns, or piers resembling pilasters, into three lights, the middle one of which is usually wider than the others, and is sometimes arched."

By "lights" the author means window panes, or the area where daylight may enter an interior space. By "sometimes arched," the author is describing the Diocletian type of Venetian window.

The Penguin Dictionary of Architecture also leads the reader to an entry other than Diocletian window.

Thermal Window. A semicircular window divided into three lights by two vertical mullions, also known as a Diocletian window because of its use in the Thermae of Diocletian, Rome. Its use was revived in the C16 [16th century] especially by Palladio and is a feature of Palladianism.

Where does the name "Diocletian" come from?

Diocletian comes from the Roman Emperor Diocletian (c. 245 to c. 312), who built the most opulent public baths in the Roman Empire (view photo). Built around 300 AD, the facility was large enough to  accommodate 3000 patrons. The Baths of Diocletian, also known as Thermae Diocletiani and Terme di Diocleziano, expanded Vitruvian ideals of symmetry and proportion. What we know today as Diocletian windows exemplify Early Fourth Century AD Classical Architecture.

The designs found in the Baths of Diocletian in Rome have been influential to architects of neo-Classical buildings and pavilions for centuries. First popularized by Andrea Palladio in the 16th century, the Roman Baths are said to have influenced Thomas Jefferson's 19th century design of The University of Virginia.

In addition to the Roman baths, Diocletian is also known to have ruled over a military camp in the Syrian city of Palmyra. The Camp of Diocletian has been a known part of the the Ancient Ruins in Palmyra.

What does Palladio have to do with Diocletian windows?

After the darkness of the Middle Ages, the Renaissance architect Andrea Palladio (1508-1580 AD) studied and revived many Greek and Roman architectural designs. To this day, our use of Palladian windows can be traced to Palladio's redesigned windows from the Baths of Diocletian.

Other Names for a Diocletian window:

  • thermal window (from Thermae Diocletiani)
  • Venetian window

Examples of Diocletian windows:

  • Interior spaces of Union Station, Washhington, DC
  • Mansion House in Ashbourne, UK has been remodeled with a Diocletian window AND a Palladian window over the the front door
  • Many of Palladio's buildings, such as San Giorgio Maggiore in Venice, Italy
  • Many Neo-Palladian (or Palladian Revival) estates, such as Chiswick House in West London, UK.

About Chiswick House:

Claiming to be "the first and one of the finest examples of neo-Palladian design in England," Chiswick House west of the City of London was designed to pay tribute to the Italian architecture of Palladio. The project began when the third Earl of Burlington, Richard Boyle (1694-1753), toured Italy and was struck by its Renaissance architecture. When he got back to England, Lord Burlington embarked on this "bold architectural experiment." Apparently, he never intended to live in the villa. Boyle instead designed "a grand pavilion where he could display his art and book collection and entertain small groups of friends." Take note of the Diocletian window in the dome area of Chiswick. There are actually four such windows bringing daylight to the interior of the octagon. Chiswick House, completed in 1729, is open to the public for tours of the house and gardens.

Learn More:

  • Chiswick House & Gardens by Roger White, Guidebook from Historic England Press, 2001

Sources: Dictionary of Architecture and Construction, Cyril M. Harris, ed., McGraw- Hill, 1975, p. 527 "Thermal Window," The Penguin Dictionary of Architecture, Third Edition, by John Fleming, Hugh Honour, and Nikolaus Pevsner, Penguin, 1980, p. 320; About Chiswick House, Chiswick House and Gardens; The Architecture of the University of Virginia by Lydia Mattice Brandt, Virginia Foundation for the Humanities; National Roman Museum - Baths of Diocletian, Soprintendenza Speciale per il Colosseo, il Museo Nazionale Romano e l'Area Archeologica di Roma [accessed March 18, 2016]