Your Local Service Center:
Vortex Logo

What is a Mullion?

March 19, 2025 | Commercial Doors

What is a Mullion?A mullion is a vertical bar that divides a door or window frame into separate sections. Invented during the Middle Ages, they were a common design element in Renaissance, Victorian, and Gothic architecture. Mullions lend structural support to large windows and double doors. While Medieval and Gothic Cathedrals used stone mullions, modern mullions are generally made from wood, steel, or aluminum. Wood mullions are typically found in residential settings, while steel and aluminum mullions are normally installed in government and commercial buildings, due to their enhanced durability. There are several types of door mullions.

  • Fixed. Permanently separate two door panels. They not only provide stability, but strength. A steel or aluminum mullion allows doors to resist winds and impact far better than they would on their own.
  • Removable. A useful feature in high-traffic areas, removable mullions can be taken out in order to widen the doorway, allowing you to accommodate oversized objects and large groups of people.
  • Decorative. Some buildings, such as hotels, use mullions to improve the aesthetic appeal of their property. Adding a mullion to French doors or glass-panel double doors can lend your building a traditional or cottage atmosphere.

Mullions are often confused with muntins and transoms. A muntin is a small strip of wood or metal that separates panes of glass into a grid or grille, while mullions divide whole window and door units. Modern muntins are often used decoratively, but in older architectural styles, they ensured the window’s weight was distributed evenly across the frame. Transoms, on the other hand, are horizontal beams that sit above doors, usually to support a glass window over the doorway.

Role of Mullions in Double Doors

Mullions provide several key benefits for commercial and industrial doors. Depending on their location, they may provide:

  • Stability. Mullions create a solid frame that double doors can lock into. This reduces pressure on the hinges and prevents doors from warping under their own weight, thereby extending their lifespan.
  • Protection. A secure anchor makes it far more difficult for intruders to force their way through a double door. It’s an additional barrier they have to overcome before they can access your property.
  • Weatherproofing. Installing a mullion in a set of exterior doors creates a tighter seal that blocks moisture and reduces energy loss, making your building more efficient.

Mullions perform a critical role in terms of both security and function. By strengthening doors, they reinforce security while reducing weaknesses that could lead to greater long-term costs.

Commercial Door Maintenance and Repair

Vortex service teams are trained to identify issues in all aspects of your doors, including mullions. We can detect when they’re misaligned, warped, corroded, or simply too worn to provide reliable support. We tighten screws, treat rust, and replace degraded components to ensure your doors continue to work as intended. Keep your business secure. Contact us today to learn more about our repair and maintenance programs.

Want to SHARE this?

Fully Certified & Accredited Commercial Door Experts

IDEA logo
Lead Safe EPA logo
ISN RAVS Plus Participant Logo
Newsweek most loved 2024
AAADM logo
NFPA logo
Avetta logo

What Our Customers Are Saying

Excellent
Eric S.
Today Tray and Johnathan with Vortex, showed up to repair our broken entrance gates. They were on time and very professional. After checking the damage, they took the time to clearly explain what needed to be done and how they would do it. Watching them while they were doing the repairs, they would measure twice and cut once'if you know what I mean, very professional! When they were done, the gates are operating perfectly, I'm very satisfied with their work!
Barbara C.
We've been long time customers with Vortex. Matt did a great job repairing our commercial glass entry doors and adjusting the closers.

The office staff did a good job handling my call and I was able to get a repair appointment next day! Service completed well within time frame expected.
So glad to see Matt come back- he helped us last time and did another terrific job. Thanks Vortex!!
Lia X.
Great work from Greg!
Kelly N.
We've had an ongoing problem with our main entrance doors and the ability to program them with our access control system. Juan arrived this morning and was able to troubleshoot the problem and offered expert advice on how to fix the problem. Juan was courteous, friendly and professional. He also went above and beyond to locate the problem. We have used Vortex several times for our building and always had great service. Juan is awesome and deserving of a 5 star review!
Valeria H.
Amazing Sevice ! Very professional and respectful. Did a great job .
Priscilla C.
Josh and Adam fixed doors for us at my job and did a great job!
Tina D.
Connor repaired the door in a timely manner. Great job!!
Jay G.
Great service from saul. knows what he's doing. Good explanation on what exactly he did and why.
Dwayne M.
Dakota came by and took great care of our doors! Awesome service and explained what and why he was doing what we needed. Very thorough and friendly.
Vortex Doors - since 1937

Perfect service, incredibly fast.

Please call us at (800) 698-6783 for 24/7 customer support.

Contact Us