Mullion

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A mullion is a vertical bar in a window or screen that divides adjacent panes. Its main job is to provide rigid support for the glazing and to transfer wind and weight down into the building. A horizontal member above the opening is called a transom.

In buildings with several arches, mullions can link openings under a single arch, forming features like bifora or trifora. Stone mullions were used in Armenian, Saxon, and Islamic architecture before the 10th century and later became common in Europe in the early Middle Ages and Romanesque architecture. They also frame open arcades, galleries, porticoes, and cloisters. In Gothic cathedrals, windows grew taller and slimmer (lancet windows), and the walls between them were reduced to slender mullions with decorative tracery. Stained glass was set in lead between the mullions and tracery.

Mullions continued in Renaissance and revival styles. A window divided into two by a single mullion is called biforate (or bifore in Italian). Mullions can be made of wood, aluminum, or even glass in some designs.

Mullions are often confused with transoms or muntins. In US usage, muntin (the UK term) refers to the small bars that subdivide a sash into panes; the mullion is the vertical structural member. In curtain walls, mullions may only support the weight of the transoms and glazing, with weight carried from above, putting the mullions under tension rather than compression.

Before the mid-19th century, very large glazed areas required a framework of mullions and transoms—often stone—and each pane was further subdivided by muntins or lead came because large glass panes were expensive. Today mullions and transoms are still used with divided-light windows for large openings.