Illustrated Architecture Dictionary

Transom /
Transom bar

A crosspiece separating a doorway from a window or fanlight above

The horizontal member across the top or middle of a window

Transom light / Transom window / Transom: In the United States, a window or pane above a door, whether rectangular or arched; also, a window that is hinged along its top edge

Light: In a window, the openings between mullions; more generally, panes

Fanlight: Arched window or pane above a door

Overdoor: An ornamental painting, carving or section of woodwork directly above a doorway - SIMILAR TO A TRANSOM WINDOW, BUT NOT MADE OF GLASS

Rectangular transoms and side lights are found in Greek Revival, Neoclassical styles

Before electric lighting, building interiors were often quite dark, even in the middle of the day. Placing transom windows above interior doors allowed light to penetrate further into the building. They are regaining popularity today as more homeowners prefer natural, rather than artificial, light in their homes. Homeowners also appreciate the security value of transom windows, which let in light even while the door below remains securely locked.

Some transom windows are fixed and cannot be opened. These windows are often decorative in some way. They may be divided into multiple panes by vertical muntins, or they might be filled with stained glass. The decorative value of transom windows was greatly appreciated in the Victorian period.

Other transom windows can open. Sometimes the hinges are placed at the bottom of the window, so that the transom window opens at the top. In this arrangement, a lightweight chain is often attached so that the window cannot fall too far. These windows can also be hinged at the top, so that they open at the bottom.

- Wisegeek  (online March 2019)

Examples from Buffalo:

Other examples:

Photos and their arrangement 2002 Chuck LaChiusa
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