A horizontal crossbar above a door or window, or a small window above a door.
From Old French 'traversin' meaning 'crossbeam' or 'bolster,' derived from Latin 'transversus' meaning 'turned across.' The word literally means 'something placed across' — trans (across) + versus (turned). Originally used in shipbuilding for the flat surface across the stern of a boat, it transferred to architecture for any horizontal beam that crosses an opening. The small window meaning came later, as these crossbeams often held decorative glass panels.
Transoms started on ships before they decorated your front door! The word originally described the flat back wall of a boat (the stern), and sailors brought the term ashore to describe any horizontal beam that crosses an opening. Those little windows above old-fashioned doors are named after the back ends of sailing ships.
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