A mullion is a framing element which divides adjacent window, door, or glass units. Mullions may be made of any material, but wood and aluminum are most common, although stone is also used in windows. Mullions are vertical and are often confused with transoms, which lay horizontally. The term is also properly applied to very large and deep structural members in many curtain wall systems.
A mullion acts as a structural member, and it carries the dead load of the glazing and the wind load acting on the glass to the anchor point and back to the building structure. The term "mullion" is also sometimes applied to the very small strips of wood or metal that divide a single sash or light into smaller panes of glass, but the word "muntin" (or "glazing bar" in the UK) is a more precise word for these small members.
In traditional designs today, mullions and transoms are normally used in combination with divided-light windows and doors when glazing porches or other large areas.