What is a mullion commonly used for in architecture?

A mullion is primarily used to form a division between window or door units. Typically, it serves as a vertical or horizontal structural element that separates and provides support for multiple windows or doors in a larger frame. This division is particularly important in architectural design as it allows for a cohesive look while enabling the structural integrity of the window or door units in terms of load-bearing capacity and stability.

While some of the other options might indicate supplementary functions that a mullion could have, such as adding aesthetic appeal or possibly contributing to thermal insulation partially, the core functionality is centered around creating divisions in fenestration. This serves both practical purposes in the design—organizing the light and views that pass through the window or door assembly—and maintaining the integrity of the overall structure.