A mullion is a major structural vertical or horizontal bar that divides or joins together adjacent window units or door panels. In addition to holding assemblies rigid against wind and weight, it supports the structure (like an arch or lintel) above the opening.
A mullion is a vertical or horizontal structural element that divides and supports adjacent units of windows, doors, or glass curtain walls. Its primary functions are to provide rigid support to the glazing, stabilize the surrounding wall, and bear structural loads from above.
The difference between a mullion and a transom comes down to their orientation: a mullion is vertical, while a transom is horizontal. Both are structural beams or framing bars used to divide windows or doors into smaller sections for support and aesthetic design.
French casement windows, for example, are mullioned windows. Unlike muntins, which are either vertical or horizontal bars that divide a larger glass pane into smaller glass panes, mullions are larger vertical components that separate large windows into several sections for structural support.
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.
A mullion is a structural, slender vertical bar used to divide adjacent glass panes within a single window, door, or screen.
The moulding around an interior door is most commonly called door casing or door trim. In architectural terms and in many countries outside the US, this flat or decorative frame is known as an architrave.
Mullions are the vertical or horizontal bars used to structurally support or divide glass panes in windows, doors, and large glass storefronts. They are categorized by how they are oriented, their functionality, and where they are placed within the frame.
No, mullion windows are not outdated. They are still widely used in modern architecture, especially for large window walls and multi-panel glass systems that need structural support and a clean design.
Yes, you can remove window mullions—but it depends heavily on whether they are decorative grids or structural framing. The methods vary based on how the mullions are attached:
Those windows above doors are called transom windows. They were essential architectural features in older homes for two main reasons:
Mullions and muntins are terms for vertical supports in windows. Muntins separate multiple panes of glass, whereas mullions divide two panes. Both terms are now less common as they no longer support glass structurally. Today, the decorative pieces are often called "grilles."
A Palladian window (also known as a Venetian window) is a large, symmetrical window divided into three parts. It features a wide, arched central section that is flanked by two smaller, narrower, square-topped rectangular sections.
A mullion is a structural bar that separates two window units or sections within a window frame. Mullions provide support and stability, allowing large window openings to be divided into smaller sections. They are commonly found in: Bay windows.
While mullions can add a level of aesthetic appeal and provide additional support, they are not necessary for all types of windows. Many modern homes prefer windows without mullions for a cleaner, unobstructed view.
Frame: The outer structure that holds the entire window assembly. Sash: The moveable or fixed framework that holds the glass. Glass (or glazing): The transparent pane(s) that let in light while insulating your space. Hardware: Operating mechanisms like locks, lifts, cranks, and balances.
Mullion thickness (the depth or face width of a vertical or horizontal framing bar) varies widely by application. Standard window and storefront mullions usually measure 1” to 2” in width and 2” to 6” deep. Commercial curtain wall mullions are bulkier, ranging from 2” to 6” wide and 4” to 10” deep to support heavy glass modules.
Outdated homes usually suffer from heavy thematic trends, worn-out finishes, or poor lighting. To elevate your space, it helps to focus on replacing dated fixtures, fixing poor lighting, and eliminating overly specific decor.
Trending Window Treatment Ideas for Every Room
A mullion is a vertical or horizontal element that forms a division between units of a window or door. Mullions provide structural support and can be made from various materials like wood, metal, or vinyl. They are different from muntins, which divide glass panes within a window sash.
Mullions are installed by cutting the mullion bar to fit the rough opening, anchoring it to the head and sill using mounting clips, and sealing the joints with silicone. This connects multiple window or door units side-by-side or stacked, ensuring the entire structure is properly aligned, structurally supported, and watertight.
Methods for repairing stone mullions
Cracked mullions and transoms can be resin filled and re-built with supporting dowels. However, if you believe that your windows have seen better days, or are tackling larger areas of deterioration, sections of stone may be totally replaced with matching stone from a quarry.
Rubber seals are commonly installed around entry doors, garage doors, and window frames to improve insulation and reduce outside interference. Rubber products like door gaskets help prevent air leaks, noise, and debris from entering indoor spaces.
A door canopy is the general name for a small roof-like structure installed above a front or back door. Its main job is to protect you from the weather while adding a decorative touch to your entrance. Door canopies come in a variety of styles, materials, and sizes.