mullion

Mullion probably derives from the Old French moinel or moynel, from moien, meaning 'middle' (from Latin medianus). The mullion is a vertical structural element that divides a window into two or more sections called 'lights.' In Gothic architecture, mullions were not just structural—they were aesthetic, branching at the top into the flowing stone tracery that defines the Gothic window.

The earliest Gothic mullions appeared in the 12th century in the Île-de-France, as builders discovered that pointed arches allowed them to open up walls with larger windows. A single large opening needed internal support, and the mullion provided it. The stone bar carried the weight of the tracery above and transferred it to the sill below. Engineering made the ornament possible.

The evolution of mullion design tracks the phases of Gothic architecture. Early English Gothic (1180-1275) used simple lancet mullions. Decorated Gothic (1275-1380) introduced flowing, curvilinear tracery above the mullions. Perpendicular Gothic (1380-1520)—unique to England—straightened the mullions into a grid pattern, creating the characteristic rectangular panels of windows like those at King's College Chapel, Cambridge, completed in 1515.

Modern architecture largely abandoned the mullion when steel and reinforced concrete made large unsupported glass walls possible. But the word returned with curtain-wall construction—the aluminum or steel bars that hold glass panels on skyscrapers are technically mullions. The medieval stone bar and the modern aluminum extrusion serve the same purpose: holding glass in place. The material changed. The function did not.