A mullion is a vertical structural element used to separate windows, doors, or other openings in a wall or frame. They provide support for the glazing and divide the opening into smaller sections.

Mullions can be made from a variety of materials, including wood, metal, fiberglass, and composite materials. The material choice depends on the specific application, environmental conditions, and aesthetic preferences.

Mullions come in various types, such as fixed, operable, curved, and thermally insulated. Fixed mullions are permanently installed, while operable mullions allow for the opening and closing of windows or doors. Curved mullions are used to create curved facades, and thermally insulated mullions minimize heat transfer between the interior and exterior.

Mullion installation involves precise measurements, cutting, and assembly. It typically requires specialized tools and expertise. Mullions can be installed in new construction or as part of renovation projects to update the look and functionality of an existing building.