- 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
Civil engineers can lead the way.
Cutting-edge tech pairs with building management systems.
BSRIA updates its assessment of the industry.
What happens when it all goes wrong?
Input being gathered by CIOB.
Changes proposed for MHCLG consultation on house building statistics.
Full of passion and acerbic wit. 1 min book review.
Reminding us what is possible.
Five signs you are at risk.
Biotechnology as it applies to the built environment.
Stopping sound coming through windows.
Government response to the Building a Safer Future consultation.
Energy savings quickly payback any small additional capital investment.