Mullion

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 often teamed with horizontal elements called "transoms" which divide an opening's upper part into one or more additional lights.

In the commercial door industry, the term "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. Italian windows with a single mullion, dividing the window into two equal elements are said to be biforate, or to parallel the Italian, bifore windows.

Mullions may be made of any material, but wood and aluminum are most common, although glass is also used between windows.I.M. Pei used all-glass mullions in his design of JFK Airport's Terminal 6 (National Airlines Sundrome), 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".

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Wikipedia