Transom.
The horizontal structural member in a curtain wall grid, spanning between mullions.
A transom is the horizontal structural member of a curtain wall, window or door grid. Transoms span between mullions, divide the bay into glazed panels (vision area) and spandrel panels (the zone covering the floor slab), and carry the dead load of the glass above them down to the mullions.
Transom depth is sized to the mullion spacing, the weight of the glass it supports, and the impact load. On a typical commercial unitised curtain wall the transom face width matches the mullion sightline for a consistent grid reading. The transom-mullion junction is one of the critical detailing points for weathertightness and is where most CWCT water test failures originate.