transom

Transom enters English from Old French traversin (a crossbeam), from Latin transversus (turned across), from trans- (across) and vertere (to turn). A transom was originally a horizontal crossbar — any beam that ran across an opening. In architecture, the transom became specifically the horizontal bar at the top of a door or window frame, and by extension, the small window above that bar.

Transom windows were practical. In the era before air conditioning (which is most of human history), a closed door with a transom window above it provided privacy with ventilation. Air could circulate over the transom while the door remained closed. Hotels, offices, apartment buildings, and hospitals all used transom windows. The phrase 'over the transom' — meaning unsolicited, as in a manuscript slipped over the transom into an editor's office — comes from this architecture.

The transom window declined with the spread of air conditioning in the mid-twentieth century. Sealed buildings did not need transom windows for ventilation. New construction omitted them. Existing transoms were sometimes sealed shut or covered over. The window that had ventilated buildings for centuries became unnecessary in a single generation.

Transom windows have returned in contemporary architecture, partly for aesthetics (they add visual height to a doorway) and partly for natural ventilation in energy-conscious design. The passive ventilation technique that air conditioning replaced is now marketed as 'green design.' The oldest technology becomes the newest trend.