What's the purpose for putting transom windows above doors?

Transom Window--A horizontal, rectangular window placed directly above a door window. These small windows are often sectioned by trim and allow additional light to enter the home when placed above an exterior window. When placed above interior doors, the purpose of transoms is usually ornamental.

https://community.fireengineering.com/m/blogpost?id=1219672%3ABlogPost%3A609707

There's a phrase in publishing, "over the transom," which refers to a manuscript that arrives through the mail, rather than via a literary agent, and is tossed, figuratively, "through the transom" to a lowly and beleaguered reader -- who might or might or might recommend it for publication:


https://en.wikipedia.org/wiki/Transom

It's always been my impression that the primary purpose of interior transom windows was to provide ventilation, while enjoying the security of a locked or closed door. This was especially needed before whole-house air conditioning, and in high ceiling applications.

In my father's house it's another way for light to get to the upstairs landing as it's only other window is on the staircase side. When you have an interior hall with little or no window space of its own these bring in light from the rooms either side of it, as do the glass doors in my own house to the downstairs hall, which had no windows at all originally.

transom windows were placed over the doors, they were capable of opening and closing, and were used to allow air to be pulled through, usually by a large whole house fan, located in the attic. You would open the double hung windows in the room, both at the top and bottom, but just 3 or 4 inches each, then when the transom was open and the fan on, warm ceiling air would be pulled out of the room by the fan, through the transom and was replaced by cooler outside air.

the main purpose of a transom window was air movement. The window could be opened and closed. The double hung windows in the room would be opened at the top and bottom and the whole house or attic fan was able to pull warmer ceiling air from the room while pulling cooler outside air into the room.

Originally: structural.

Today ventilation / ornamental.

"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.

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 office buildings and apartments before air conditioning became common."

https://en.wikipedia.org/wiki/Transom_(architectural)