What is a window mullion?

Written by veronica james | 13/05/2017
What is a window mullion?
Mullions have been an optional architectural feature in buildings since the 12th century. (english village house image by Chris Lofty from Fotolia.com)

Modern housing has lost much of the elegance and durability of buildings of previous centuries, evidenced by cheaper building materials and strict budgets for architects planning new homes. In previous generations, mullion windows were a feature of country and village houses as well as castles and stately homes, and represent an architectural feature that dates back to at least 12th century Europe.


What is a window mullion?
Stone mullioned window and leaded lights (medieval mullioned window image by Chris Lofty from Fotolia.com)

A mullion is a vertical pillar of wood, stone or brick that divides the window areas, providing structure and support and separating the window area into compartments. Often the top of the window openings are arched or curved although they might also be straight horizontals. Mullions provide a strong foundation for the insertion of glass windows, if they exist, or simply provide an elegant feature in buildings that predate the use of glass.

Flying and floating mullion windows

What is a window mullion?
A casement window with a central mullion (Window image by annalovisa from Fotolia.com)

A flying, or floating, mullion is a modern interpretation of the mullion window. It is constructed to give the impression of a central pier, or mullion, which is attached to the edge of one or other of the two casement windows that open out on both sides. This type of window is a useful feature in a house that has windows that do not open fully, and offers the occupant a way of escape in the event of fire.

Glass panes

What is a window mullion?
Stained glass mullioned windows (stained-glass windows image by Valeri Vlassov from Fotolia.com)

Windows held within the mullions and walls are known as lights, and in historical buildings such as churches and cathedrals, mullion windows are usually stained glass. Stately homes and manor houses may have one or two windows of stained glass, but in the main, as with other types of buildings such as civic architecture or family homes built in the traditional way, clear glass and sometimes leaded lights were and still are the material of choice.


The Online Etymology Dictionary dates the word "mullion" back to the Old French "meien", meaning average, or mean. The Collins Dictionary confirms the evolution of that word to "moinel", after the Norman Conquest of the 11th century and the fusion of English with French during that time. In the medieval period, the word became "moyniel" and by the 16th century had become "mullion".

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