mullion

Imagine an old church or stone house with a large window made of several tall sections instead of one big sheet of glass. A mullion is the vertical bar between those sections. It helps support the window and also gives it a clear pattern and shape. Mullions are common in older buildings, especially in Gothic and Tudor architecture, but they also appear in some modern windows, doors, and glass walls. You can think of one as the upright part that separates and supports the opening.

A mullion is a vertical structural member that divides the space between window units, doors, or other glazed openings. Historically, these were often fashioned from stone, though wood, metal, and reinforced concrete have also been widely used. Far from being merely decorative, mullions provide vital structural support; they help hold the glazing or panels in place and can transfer loads from the upper parts of the window or the wall above down to the sill or surrounding frame.

The term is rooted in Middle English mulyon, likely evolving from an Old French word for a central post or "stump." While most famously associated with the soaring, light-filled heights of Gothic cathedrals—where intricate stone mullions branch into delicate tracery—they are a staple of many architectural styles, from Tudor manor houses to modern glass-and-steel skyscrapers. You will frequently hear "mullion" used alongside "transom," which refers to the horizontal bar that serves a similar structural purpose. Together, they form the grid that defines the window's geometry. Although modern engineering allows for vast expanses of unsupported glass, mullions remain a beloved design feature, evoking a sense of history and craftsmanship. The next time you find yourself admiring a grand stained-glass window, remember that those elegant vertical bars are mullions, faithfully supporting centuries of light and color.