The mullion — the vertical bar that divides a window into sections — takes its name from its position in the middle of the opening. The word is an alteration of Middle English monial or munnial, from Old French moienel (middle, central element), derived from moien (middle), from Latin medianus (of the middle, median). The change from monial to mullion occurred in the 16th century through a process of sound change that is not entirely clear but may involve analogy with other words.
Mullions were originally a structural necessity rather than a decorative choice. Before the development of large-scale glass manufacturing in the 17th and 18th centuries, window glass could only be produced in small panes. A large window opening required division into smaller sections, each holding a manageable piece of glass. The mullion provided the structural framework for this division, supporting both the glass and the weight of the masonry above.
In Gothic architecture, mullions achieved extraordinary sophistication. The great windows of Gothic cathedrals — from Chartres to York Minster — use stone mullions that branch into elaborate tracery patterns at their tops, creating the characteristic pointed-arch window frames that define the Gothic style. The tracery above the mullions became the canvas for some of the most complex geometric and curvilinear designs in the history of architecture.
Tudor domestic architecture made the mullioned window its signature feature. The typical Tudor window, with its vertical stone or wooden mullions and horizontal transoms (the crossbars), divided the window into a grid of small panes. The mullioned windows of Tudor manors like Hardwick Hall — famous for being 'more glass than wall' — represent the apogee of mullioned window design.
As glass technology improved and larger panes became available, mullions became optional rather than necessary. Georgian architecture favored the sash window, which eliminated mullions in favor of large sliding panes. Victorian architects, however, revived mullioned windows as part of the Gothic Revival, and mullioned windows remain a feature of many traditional architectural styles.
In modern architecture, the mullion has been reinvented as a component of curtain wall systems — the metal frames that hold glass panels on the facades of skyscrapers. These modern mullions serve the same fundamental function as their medieval predecessors: holding glass in place and transferring loads to the building structure.