Amullion is the verticalelement that forms a division between units of awindow orscreen, or is used decoratively.[1] When dividing adjacent window units its primary purpose is a rigid support to the glazing of the window. Its secondary purpose is to provide structural support to the horizontallintel above the window opening (Horizontal elements separating the head of a door from a window above are calledtransoms). In the case of multiple arched windows combined building abifora (ortrifora, etc), the equally shaped windows might be coupled by mullions (instead of acolumn with base and capital), framed by a blind arch or the mullions dissolving inGothictracery.
Stone mullions were used inArmenian,Saxon andIslamic architecture prior to the 10th century. They became a common architectural feature across Europe inearly Mediaeval andRomanesque architecture, with biforas, paired windows divided by a column or a mullion, set beneath a single arch. The same structural form was used for openarcades as well as windows, and is found ingalleries,porticos andcloisters.
Particularly inGothic cathedrals and churches, windows became higher and slender (so-calledlancet windows), the walls between them diminished to slim stone mullions building large arrangements of multiple openings with ornamental tracery in the arches. Pictorialstained glass (after designs by sculptors or painters) was set in lead and ferramenta between the stone mullions and tracery. Mullioned windows of a simpler form continued to be used into theRenaissance and variousRevival styles. Windows with a single mullion, dividing the window into two equal elements are said to be biforate, or to parallel the Italianbifore windows.[2]
Mullions may be made of any material, butwood andaluminium are most common, although glass is also used between windows.[3]I. M. Pei usedall-glass mullions in his design of JFK Airport's Terminal 6 (National Airlines Sundrome), unprecedented at the time.[4]
Mullions are vertical elements and are often confused withtransoms, which lie horizontally. In US parlance, the word is also confused with the "muntin" ("glazing bar" in the UK) which is the precise word for the very small strips of wood or metal that divide asash into smaller glass "panes" or "lights".
A mullion acts as a structural member, in most applications the mullion transfers wind loads and weight of the glazing and upper levels into the structure below. In acurtain wall screen, however, the mullions only support the weight of the transoms, glass and any opening vents. Also in the case of a curtain wall screen the weight of glazing can be supported from above (providing the structure can take the required loads) this puts the mullions under tension rather than compression.
When a very large glazed area was desired before the middle of the nineteenth century, such as in the large windows seen inGothic churches orElizabethan palaces, the openings necessarily required division into a framework of mullions and transoms, often of stone. It was further necessary for each glazed panel, sash orcasement to be further subdivided bymuntins or leadcames because large panes of glass were reserved primarily for use as mirrors, being far too costly to use for glazing windows or doors.
In traditional designs today, mullions and transoms are normally used in combination with divided-light windows and doors when glazing porches or other large areas.
Media related toMullions at Wikimedia Commons