What is a mullion in the context of windows?

A mullion refers to the vertical or horizontal dividing element between two or more windows. In the context of windows, it serves as a structural support that provides stability and can also enhance the aesthetic appeal of the window arrangement. When multiple windows are joined together in a façade, the mullion helps to delineate the individual window units.

Understanding this term is crucial in the study of exterior components since the design and placement of mullions can affect both the appearance and performance of windows in terms of energy efficiency and structural integrity. In contrast, options like a kind of window frame or a type of window blind do not accurately describe the function of a mullion, and describing a mullion as a window that cannot be opened is also incorrect, as mullions are not windows in themselves but rather components that interact with window assemblies.