Mullion

A mullion ties separate units into one larger opening. Think of a pair of windows set next to each other, or a door with a window stacked above it. The mullion carries the joint and often adds strength.

Do not confuse it with a muntin. A muntin divides glass inside a single sash. A mullion divides whole units. Mullions affect the look of a wall, so ask for the visible width early. Slimmer mullions keep more view, a point we cover in the sightline entry and across our steel windows and doors. Ask whether the mullion is structural.