Our previous Explainers have touched on some of the materials used in exterior cladding systems that give buildings their unique appearance. Curtain walls and spandrel glass are both frequently used in modern highrise construction, but there's a typical implement in a building's exterior that is often overlooked.

Stained glass window mullions supporting an archway at Salisbury Cathedral, image by Marcus Mitanis

The vertical elements that separate doors and windows are known as mullions. They can be used decoratively, but their primary purpose is to provide structural support. In the past, they were mostly utilized in buildings like churches to support brick or stone archways above the windows. Today, they're often partnered with horizontal bands known as transoms, acting as support for the building's glazing. They are also installed to accommodate window openings, usually for condominium buildings. Modern mullions are typically comprised of either aluminum, steel, wood, or fibreglass, whereas stone was the primary material pre-20th century. 

Aluminum mullions used to separate windows in a Toronto building, image by Marcus Mitanis

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