A transom is a horizontal structural beam or bar that separates a door from a window above it in architecture. It is also the customary U.S. word used for a transom light, which is the window over this crosspiece. 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. Transom windows, also called transom lights, have been around since the 14th century. They are the rectangular or semicircular windows above a door or other window, and they sit on top of a horizontal beam called a transom. Though transom windows are typically more decorative than functional, they let additional natural light into a room at a height that didn’t enable.
In addition to architecture, a transom is also a vertical reinforcement used to strengthen the stern of a boat. It is a thick, flat surface that you see at the rear of modern boats.
