The general architectural meaning
In its broadest sense, a mullion is the upright member that separates two or more panes, sashes, lites or panels set side by side in one larger opening. A window made of three glass units will have two mullions between them. A storefront or curtain wall is a grid of mullions (vertical) and transoms or rails (horizontal) holding the glass.
A mullion is usually structural: it carries load and stiffens the opening, which is why in a large glazed wall the mullions are the deeper, heavier members. That is the sense most people mean when they say mullion outside the door trade.
The Division 08 meaning: mullions in door openings
In commercial door work, mullion most often means the vertical member that runs floor to head between the two leaves of a pair of doors, or between a door and an adjacent sidelite, inside a single frame. It splits one wide opening into two narrower ones.
The mullion does real work in a door opening. It gives the inactive (normally non-operating) leaf of a pair something to latch and lock into, and it provides the strike point that an exit device on the active leaf pushes against. Without it, a pair of doors has no solid center to secure against, which is why wide openings so often have one.
- Divides a wide frame into two door openings, or a door plus a sidelite
- Carries the strike and locking hardware for the inactive leaf
- Gives exit devices a point to latch into at the center of a pair
- Can be a fixed part of the frame, or removable
Fixed vs removable mullions
A fixed mullion is welded or built into the frame and stays put. It is the simplest and most secure arrangement, but it permanently narrows each half of the opening.
A removable mullion mounts into the frame top and bottom and can be taken out with a key or lever to clear the full width of the opening, which matters when you need to move large equipment or furniture through. Removable mullions are common on fire-exit pairs fitted with panic hardware. They are a topic in their own right, covered in the removable mullions guide.
Mullion vs muntin: the common mix-up
Mullion and muntin get confused constantly, so it is worth being precise. A mullion divides whole units from each other. A muntin is the thinner bar that divides the individual panes of glass within one sash or lite, the grid you see on a traditional multi-pane window.
A simple rule: mullions separate the big pieces, muntins separate the little panes inside one of those pieces. If you are specifying or pricing commercial openings, the member you care about is almost always the mullion.
Why the name eMullion
The mullion is the quiet part that holds an opening together and carries the hardware, which is a fitting image for software built for Division 08 distributors. eMullion is a CPQ and project-management platform for the companies that supply commercial doors, frames and architectural hardware, and the name is a nod to the trade it serves.
