The stationary frame piece that separates mulled window or door units Being joined by mullions makes multiple window or door units appear to be one Not to be confused with muntin bar (View illustration )
An intermediate connecting member used to "marry" two or more windows or patio doors together in a single rough opening without sacrificing air or watertight performance A mullion also can give added strength to the connection for structural stability
A vertical post or other upright that divides a window or other opening into two or more panes Sometimes only ornamental