A vertical structural element that divides windows, doors, or other openings into multiple sections, providing structural support and architectural divisions.

A mullion is a vertical structural element that divides windows, doors, or other openings into multiple sections, providing structural support while creating architectural divisions and enabling the use of multiple glass panels or door leaves in a single opening. These essential architectural components serve both functional and aesthetic purposes, allowing for larger openings while maintaining structural integrity and creating visual rhythm and proportion in building facades. Mullions are fundamental elements in window and door design, supporting everything from residential windows to large commercial curtain wall systems.

The primary purposes include providing structural support for large window and door openings, dividing openings into manageable sections for glazing and hardware installation, creating architectural rhythm and visual interest in building facades, enabling the use of standard-sized glass panels in large openings, supporting the weight of glazing systems and transferring loads to the building structure, allowing for different operating configurations within a single opening, and facilitating maintenance and replacement of individual glazing units.

Types and classifications involve structural mullions, decorative mullions, removable mullions, and integrated mullions for different applications and performance requirements.

Fun Fact: The word “mullion” comes from the Old French “moinel,” meaning “middle”! Medieval stone mullions in Gothic cathedrals were often carved with intricate tracery patterns that became architectural masterpieces. Some modern curtain wall mullions can span over 20 feet between supports. Interestingly, the famous Chrysler Building in New York uses decorative mullions as part of its iconic Art Deco design!