Mullions and muntins are classic parts of windows that have been used for centuries, and people often mix them up. It’s easy to see why, since both can make windows look like they have a grid pattern.
Though they might sound similar, mullions and muntins serve different purposes in a window’s structure and design. Understanding the difference is key for anyone involved in a Home renovation, historical restoration, or new construction project. This guide will clarify the unique roles of each component, explore their history, and show how they are used in modern window design. By the end, you’ll be able to tell a mullion from a muntin with confidence.
Mullions vs. Muntins: What’s the Difference?
Mullions and muntins have been used in making windows and doors for hundreds of years. Both divide glass panes, but they do it in different ways. Let’s look at each one to clear up any confusion.
What Is a Mullion?
A mullion is a thick vertical or horizontal bar that separates two window or door units. You can think of it as a support beam. When you see a group of windows joined together, the bars between each window frame are mullions. They help connect several windows into one larger opening.
Millions have been around since the 10th century. They were first made from stone and used to support large windows in cathedrals and castles. (Mullion – Wikipedia, n.d.) These bars helped hold up the weight of arches above, making the big glass walls in Gothic churches possible. (Mullion, n.d.)
In modern architecture, mullions still serve the same primary function of providing structural support, though they are now typically made from wood, aluminum, or composite materials instead of stone. You’ll often see them in large glass curtain walls on commercial buildings or separating a main door from its sidelights.
What Is a Muntin?
A muntin is a thin strip of wood, vinyl, or metal that divides one pane of glass into smaller sections, making a grid pattern. To most people, a window with muntins looks like it has many small panes of glass, called “lites.”
Historically, muntins were born out of necessity. First appearing in 17th-century architecture, they were Muntins first appeared in the 17th century because it was hard to make large sheets of glass. Big panes were fragile and tricky to produce, so window makers used muntins to hold smaller pieces of glass together in one window. (Muntin – Wikipedia, n.d.) This method became so common that it shaped how windows looked for hundreds of years. (Mullion – Wikipedia, n.d.) to create large, durable sheets of glass, making the original function of muntins obsolete. However, their aesthetic appeal endured. Today, muntins are almost exclusively used for decorative purposes to replicate the classic, historic look of multi-paned windows. (What are muntins? | FAQ, n.d.)
Modern Uses for Mullions and Muntins
Even though mullions and muntins started out long ago, they have changed to fit today’s building and design needs. Their roles have shifted, but they are still important for making windows that work well and look good.
The Evolving Role of Mullions
Today, mullions do some of the structural work that muntins used to do, but for much bigger windows. They are key to building large glass walls and joining different window types. For example, a mullion can connect a picture window with casement windows on each side, or link several windows to make a bay or bow window.
Millions also contribute to a window’s aesthetic, creating strong vertical or horizontal lines that can define a building’s style. They can be made to match the window frame for a seamless look or painted a contrasting color to become a striking design feature.
Millions can also make windows more useful. By adding hardware like hinges or openers to a mullion, you can turn a large fixed window into a casement or awning window that opens for fresh air.
Decorative Muntins in the Modern Age
Since muntins are no longer needed for structure, they are now mostly used for decoration. Homeowners and designers like them for adding character and detail to windows. If you are restoring an old building, windows with true divided lites—where muntins really separate each pane—are often chosen for an authentic look.
For new buildings or window replacements, there are a few ways to get the grid look:
- Permanent Grids (Grids-Between-the-Glass): These muntins are placed between the two panes of a double-paned window. They make the window look like it has divided lites and are easy to maintain since there are no outside grids to clean.
- Removable Grids: These are usually made of wood or vinyl and snap onto the inside of the window. You can take them off to clean the glass easily.
- Simulated Divided Lites (SDLs): This choice gives the most realistic look without using real divided lites. Muntin bars are attached to both the inside and outside of the glass. Some designs also add a “spacer bar” between the panes to match up with the outside muntins for a better effect.
Muntin grids are also available in many patterns to match different architectural styles, such as:irie Grid.
- Circle Top Grid
- Gothic Grid
A Timeless Legacy
Even though mullions and muntins are among the oldest parts of window design, they have adapted well to modern times. They show how building features can change from being just practical to becoming important style elements. Whether they give support or add classic charm, both still play a big role in making windows that look good and work well today.
