When there is a window above a door, the horizontal piece of wood between them is called a transom. The first transoms were used in Gothic architecture during the Middle Ages.
Architectural transoms serve the purpose of strengthening the structure of the door. The window above is also commonly referred to as a transom. In nautical terms, a transom is either the stern (or back) of a boat or a horizontal beam that helps support the stern. It was once common to describe an unsolicited manuscript as over the transom, as if a writer had tossed it over the publisher's door.