MULLION (corrupted from " munnion "; this is derived from Fr. moignon, stump), in architecture, the English term for the perpendicular pieces of stone, sometimes like columns, sometimes like slender piers, which divide the bays or lights of windows or screen work from each other; equivalents are Fr. meneau, Ital. regolo, Ger. Fenstorpfoste. H. Wedgwood (Diet, of Eng. Elym.) points out that the mullion is " the stump of the division before it breaks out into the tracery of the window." In all styles, in less important work, the mullions are often simply plain chamfered, and more commonly have a flat hollow on each side. In larger buildings there is often a bead or bowtel on the edge, and often a single small column with a capital; these are more frequent in foreign work than in English. Instead of the bowtel they often finish with a sort of double ogee. As tracery grew richer, the windows were divided by a larger order of mullion, between which came a lesser or subordinate set of mullipns, which ran into each other.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)