- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 02 Jun 2018
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.
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.
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'.
 Find out more
 Related articles on Designing Buildings Wiki
Featured articles and news
A material with exciting potential.
ECA-partnered survey shows the clear benefits.
Hire for potential, not competence.
A single knowledge hub for global infrastructure.
Compliance in construction.
The growth of the smart homes market.
Giving professional advice to friends.
Towards a radical eclecticism.
Showing the impact of new buildings on their surroundings.
Soft Landings for refurbishment projects.
An invaluable book for everyone involved in conservation.