A transom is a horizontal crosspiece mounted above a door. Transoms are used to separate the framing of the door from the framing for windows, hinged flaps, lights, and other decorative accents which may be used over a door. People sometimes refer to such ornamentation as “transoms.” Many styles of architecture utilize transoms, and these architectural features can appear in a range of guises, ranging from ornately carved blocks of wood to simple horizontal beams.
One common reason to use a transom is to separate a door from a window which is hung above it. Transom windows, as they are known, may have a fan-like shape, which is traditional. Some are designed simply to provide light, while others may be made from stained or patterned glass to add ornamentation. In some cases, the house number or name of the building may be integrated into the transom window. In the interior of a structure, the window provides light while maintaining privacy, as the door can be left solid.
Devices over doors which open to provide ventilation are also separated from the door itself with a transom. These devices can be very useful in situations when people do not want to leave the door wide open, but do want to create a cross breeze inside a structure. They have also inspired the phrase “over the transom” to describe material which reaches someone by unconventional means, referencing the idea that the material has literally been thrown over the transom into someone's home or office.
A transom light or fan light on a door which admits people into a building can also be mounted with a transom. Lighting over doors is extremely common for aesthetics and safety in many regions of the world, with the light being used to help people inside a structure identify people at the door, to provide light for opening the door in the dark, and to deter criminals who might lurk around a dark doorway waiting for victims.
The framing of a door is independent from a transom or any other decorative features which may surround the door. Transoms do provide additional support, but they primarily serve as a point of transition for architectural features. Because they are not necessarily structural in nature, transoms can be made from a wide variety of materials, and they may include decorative features which could not be used on structural supports due to concerns about fragility.