Mullion (and transom)

A mullion is the vertical structural bar that divides a window, door, partition or facade into separate sections or panes; its horizontal counterpart is called a transom. In an aluminium framing system they're not just visual dividers — the mullions and transoms are the structural grid that carries wind and glazing loads and holds each pane or infill panel in place, which is why they matter on aluminium entrance doors, office/room partitions, shopfronts and curtain-walling. Their size and spacing are engineered to the span and load, and on external systems a good mullion has a thermal break so heat and condensation don't transfer straight through the metal. Slimmer sightlines (thinner mullions) look sleeker but must still meet the structural requirement. For a framed door, partition or facade designed and made to spec, an aluminium fabrication and design specialist sizes the mullions and transoms to the job.