Door of 10 Downing Street, London

In architecture, a transom is a transverse horizontal structural beam or bar, or a crosspiece separating a door from a window above it. This contrasts with a mullion, a vertical structural member.[1] Transom or transom window is also the customary U.S. word used for a transom light, the window over this crosspiece.[1][2] In Britain, the transom light is usually referred to as a fanlight, often with a semi-circular shape, especially when the window is segmented like the slats of a folding hand fan. A prominent example of this is at the main entrance of 10 Downing Street, the official residence of the British prime minister.


In early Gothic ecclesiastical work, transoms are found only in belfry unglazed windows or spire lights, where they were deemed necessary to strengthen the mullions in the absence of the iron stay bars, which in glazed windows served a similar purpose. In the later Gothic, and more especially the Perpendicular Period, the introduction of transoms became common in windows of all kinds.[3]


Transom windows which could be opened to provide cross-ventilation while maintaining security and privacy (due to their small size and height above floor level) were a common feature of apartments, homes, office buildings, schools, and other buildings before central air conditioning and heating became common beginning in the early-to-mid 20th century.[4][5]

In order to operate opening transom windows, they were generally fitted with transom operators, a sort of wand assembly.[6] In industrial buildings, transom operators could use a variety of mechanical arrangements.[7]

Idiomatic usage

The phrase "over the transom" refers to works submitted for publication without being solicited. The image evoked is of a writer tossing a manuscript through the open window over the door of the publisher's office.[8]

Similarly, the phrase is used to describe the means by which confidential documents, information or tips were delivered anonymously to someone who is not officially supposed to have them.[9]

Some such phrases may refer instead to the transom of a ship - large waves from behind can bring water over the transom.

A ranma found in Kōchi Castle designed to look like a wave.

"Like pushing a piano through a transom" is a folk idiom used to describe something exceedingly difficult; its application to childbirth (and possibly its origin) has been attributed to Alice Roosevelt Longworth and Fannie Brice.


In French, transom windows are called vasistas (previously spelled wass-ist-dass), from the German was ist das?; lit. "what is that?".[10][11]


Architectural details called ranma (欄間) are often found above doors in traditional Japanese buildings.

These details can be anything from simple shōji-style dividers to elaborate wooden carvings, and they serve as a traditional welcome to visitors of the head of the household.

See also


  1. ^ a b "The Efficient Windows Collaborative: Glossary". Archived from the original on 2007-11-28. Retrieved 2007-12-13.
  2. ^ "What is a transom window?". Big Blue Window. Archived from the original on 2007-12-28. Retrieved 2007-12-13.
  3. ^ "The Principles of Gothic Ecclesiastical Architecture, Elucidated by Question and Answer, 4th ed". Project Gutenberg. Archived from the original on 2011-06-25. Retrieved 2007-12-13.
  4. ^ Fred. T. Hodgson, "Ventilation of Middle-Class Dwellings", Popular Science News August 1902, p. 185
  5. ^ "Going 'over the transom': Interior Windows and the Hardware that Moves them", Old-House Journal January–February 1996, p. 52
  6. ^ Brian D. Coleman, "Window Hardware 101" Old House Interiors July–August 2010, p. 29
  7. ^ Lord & Burnham Company, Manufacturers of Sash Operating Apparatus for Hinged and Pivoted Sash, in Chemical Engineering Catalog, 1919, p. 706
  8. ^ "What Does Over The Transom Mean?". About Freelance Writing. Archived from the original on 2007-12-13. Retrieved 2007-12-13.
  9. ^ Hartocollis, Anemona. Former G.O.P. Official Admits He Evaded Taxes Archived 2017-11-05 at the Wayback Machine, The New York Times, 16 November 2007. DA Morganthau Cites "Over the Transom" Letter as root of fraud investigation
  10. ^ vasistas” in Trésor de la langue française informatisé (The Digitized Treasury of the French Language).
  11. ^ Grammarphobia blog: vasistas