Transom

Transom (pronounced tran-suhm)

(1) In architecture, a crosspiece separating a door or the like from a window or fanlight above it.

(2) In glazing, a window above such a crosspiece, also called transom light, transom window or fanlight.

(3) A crossbar of wood or stone, dividing a window horizontally.

(4) A window so divided.

(5) In naval architecture, a flat termination to a stern, above the water line.

(6) In shipbuilding, a framework running athwartships (from one side to the other of a vessel at right angles to the keel) in way of the sternpost of a steel or iron vessel, used as a support for the frames of the counter.

(7) In the design of artillery pieces, a metal piece connecting the sidepieces of the tail or the cheeks of a gun carriage.

1325–1375: From the late Middle English traunsum and traunsom, an evolution of the Middle English transyn (crossbeam spanning an opening, lintel).  It was probably dissimilation from the Latin trānstrum (crossbeam, especially one spanning an opening), the construct being trāns (from the primitive Indo-European terh (through, throughout, over) and tere (cross over, pass through, overcome) which was cognate with the English through, the Scots throch (through), the West Frisian troch (through), the Dutch door (through), the German durch (through), the Gothic þairh (through), the Albanian tërthor (through; around) and the Welsh tra (through) + trum (the instrumental suffix).  The Old French traversin (crosspiece) was derivative of travers (breadth) and drawn from the Middle French traversayn and the Old French traversing.  The meaning "small window over a door or other window" was first recorded in 1844.

Windows

In architecture, used for both external and interior work, a transom is a transverse horizontal crosspiece separating a door from the window above and, in modern application, may be structural or merely decorative.  A similar device, if installed vertically is a mullion.  The adoption in England of the more evocative synonym fanlight followed the early Victorian fashion of forming transom windows in a semi-circular shape, the ribs segmenting the slats to resemble a folding hand fan.

Although now almost always a decorative affectation, the origin lies in architectural necessity at a time when structural engineering was less advanced.    In early Gothic ecclesiastical work, transoms were used in the unglazed windows of belfries or spires where they strengthened the mullions in the absence of the iron stay bars of glazed windows.  In the later Gothic, and especially the Perpendicular Period, transoms became common in many designs of windows, whether or not a structural need.

Lindsay Lohan at the stage door of the Playhouse Theatre London, June 2014 for rehearsals for Speed-the-Plow (1988) by David Mamet (b 1947).  This is a two-panel transom.

Front door of the UK prime minister's residence at 10 Downing Street.  This style of transom window is called a fanlight.