Last edited 01 Apr 2021


Mullions are members that form vertical divisions between units of a window, door, screen or glass curtain wall. Together with horizontal members known as transoms they provide rigid support to glazing.

A mullion wall is a structural system whereby the load of the floor slab is taken by prefabricated panels around the perimeter.

They can be used for decorative purposes, or to allow the combination of smaller window units into larger compositions, or to provide structural support to an arch or lintel. They can also be used as an element in door frames to divide an opening and allow two separate doors to be accommodated by a single frame.

The use of mullions dates back to before the 10th century. As the use of glass for windows became widespread, so did the use of mullions, as they allow larger openings to be sub-divided into smaller windows which are technically easier to manufacture and less expensive.

They can be found in Armenian, Saxon and Islamic architecture. They became more commonly used to divide paired windows in Romanesque architecture, as well as open arcades. The use of stone mullions in Gothic architecture increased both in terms of size and complexity, as they were used both structurally and ornamentally, particularly in churches for stained glass windows.

Modern mullions are commonly made from materials such as timber, aluminium, steel and UPVC.

The word 'mullion' is also commonly used to describe vertical members between panes of glass within a window assembly (rather than between window units). These smaller members are sometimes referred to as 'muntins'.

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