What is a mullion in the context of architecture?

A mullion refers specifically to a vertical member that is used between the panels of a window, door, or other openings in a building's facade. It serves both structural and aesthetic purposes, creating divisions between adjacent units, such as window panes or door sections, and allowing for greater stability in larger openings.

In addition to its structural benefits, the mullion enhances the visual appeal of the architectural design by providing a framework that delineates the different sections of the window or door. This is particularly important in modern architecture, where large expanses of glass are often used.

The other options describe different architectural elements that do not apply to the definition of a mullion. For instance, a horizontal support beam relates to the overall structural integrity of the building but does not share the same function or placement as a mullion. Decorative features on facades may enhance the appearance but do not play a structural role like a mullion. Meanwhile, types of roof assemblies pertain to roofing techniques and are unrelated to the vertical components of openings.