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A mullion is a vertical element that forms a division between units of a window, door, or screen, or is used decoratively. When dividing adjacent window units, its primary purpose is to provide structural support to an arch or lintel above the window opening. Its secondary purpose may be as a rigid support to the glazing of the window. When used to support glazing, they are teamed with horizontal supporting elements called "transoms".
In the commercial door industry, the term is "floating mullion" is also applied to an interlock profile which is fitted in between a pair of double swing doors.
Stone mullions were used in Armenian, Saxon and Islamic architecture prior to the 10th century. They became common across Europe in the Romanesque architecture, with paired windows divided by a mullion, set beneath a single arch becoming a fashionable architectural form. The same structural form was used for open arcades as well as windows, and is found in galleries and cloisters.
In Gothic architecture windows became larger and arrangements of multiple mullions and openings were used, both for structure and ornament. This is particularly the case in Gothic churches where stained glass is set in lead and ferramenta between the stone mullions. Mullioned windows of a simpler form continued to be used into the Renaissance and various Revival styles.
Mullions may be made of any material, but wood and aluminum are most common, although glass is also used between windows. I.M. Pei, in his design of JFK Airport's Terminal 6 (National Airlines Sundrome) used all-glass mullions, unprecedented at the time.
Mullions are vertical elements and are often confused with transoms, which lie horizontally. The word is also confused with the "muntin" (or "glazing bar" in the UK) which is the precise word for the very small strips of wood or metal that divide a sash 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. Although in the instance of a curtain wall screen, the mullions only support the weight of the transoms, glass and any opening vents installed. 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 in Gothic churches or Elizabethan 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 or casement to be further subdivided by muntins or lead cames 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.
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