A vertical structural element that divides a window or door opening into sections, typically supporting the glass panels above. Mullions can be structural or purely decorative, defining the rhythm and scale of fenestration.
From Middle French 'moyen' (middle) through Old French 'moillon,' entering English in the 14th century. Originally referred to the central post of Gothic windows that allowed for larger openings while maintaining structural integrity.
Gothic cathedrals pushed mullions to their artistic peak - look at Sainte-Chapelle's windows where stone mullions become delicate tracery that seems to dissolve into pure light and color. Modern curtain walls still use mullions, but now they're sleek aluminum extrusions that create the grid patterns defining contemporary office buildings.
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