- Project plans
- Project activities
- Legislation and standards
- Industry context
- Specialist wikis
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.
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'.
 Related articles on Designing Buildings Wiki
Featured articles and news
ECA and SELECT offer assistance to members set back by delays.
The virtual learning event examines Historic Places - People Places.
Getting post-pandemic infrastructure on the right track.
One of England's grandest country houses.
Take just two minutes to provide your feedback.
An update of standards and regulations is under consideration.
Exploring the key to the adoption of this abundant energy source.
His clients have ranged from Liberace to St Nick to world-class athletes.
These tactical structures can be permanent or temporary.
Organisation recognises milestones of the project's next phase.
Welding and metalworking businesses must manage respiratory risks.
New report explores how regulations are being put into action.
The golden thread and BS 8644-1.