Mullion.
The vertical structural member in a curtain wall grid that spans between floors.
A mullion is the vertical structural member of a curtain wall, window or door grid. In a curtain wall, mullions span between floor slabs and are fixed back to the structure via engineered brackets at the head and base. They carry the wind load on the facade and transfer it back to the primary structure.
Mullion depth (typically 90mm to 250mm) is sized to the span between supports, the wind exposure, and the deflection limit (usually L/200 or 20mm, whichever is smaller). Sightline (face width) is typically 50mm or 60mm and is one of the most important architectural decisions on a curtain wall — slim sightlines look elegant but reduce thermal performance because there is more frame per m² of facade.
On thermally-broken mullions, a polyamide isolator (typically 24–34mm) separates the internal and external aluminium chambers to break the thermal bridge.