Window Mullions vs. Window Muntins

Window muntins and mullions often get confused because they sound alike. But what are these features that give windows the grid-like, multi-paned look? And are they related to window stiles and grilles?

Understanding the word origin means going back a few hundred years to England, where both terms originated. For the most straightforward explanation, muntins are the vertical pieces of wood separating panes of glass in a classic multi-pane glass assembly. Meanwhile, mullions are the single vertical supports used in two-pane assemblies.

Here's more about window muntins, mullions, grilles, and stiles and what you need to know about them, especially if you intend to get some old-fashioned-looking windows.

Window Muntins

Even though muntins are most frequently associated with paned windows, they can mean any vertical divider, whether for windows, wood panels, furniture, or doors. Early builders used muntins because they were structurally necessary. Outer walls could not carry the weight of large windows when placed in them, and muntins allowed weight to be transmitted vertically.

For example, this was the case with Westminster Abbey's vast fenestration: Its west window is close to 45 feet high.

When searching for the word's use in history, as far back as 1688, the writer R. Holme referenced "moontans and panels." Another writer, T. Smith, referenced "mountaynes" as being "six pieces of timber...ready prepared for [Westminster] chapel." "Mountayne" is a Middle English word meaning peak, mountain, or pile.

The clearest early definition comes in 1850, from an English architectural glossary referencing "English joiners [who] apply the term 'muntin' to the intermediate upright bars of framing, and call the outside uprights styles." Styles are usually expressed today as "stiles."

Window Mullions

Mullions are different from muntins, though they both act as supporting devices. Another way to think of them is that mullions are often heavier elements that divide larger window frames. Sometimes, they can appear to create two windows where one window would be expected.

The Oxford English Dictionary defines a mullion as "a vertical bar between the panes of glass in a window." Before the Victorian Age and the advent of inexpensive plate glass, manufacturing large sheets of glass was impossible or prohibitive. Giant expanses of windows were achieved by holding smaller panes of glass together with supportive devices.

Two separate windows that are closely mated can even have that thin separating element referred to as a mullion, even though it is technically part of the wall.

The Decline of Paned Windows

After World War II, mullions and muntins fell out of favor. With larger expanses of tempered plate glass available on the residential market, small panes separated by supports were no longer necessary. Muntins and mullions became old-fashioned and unnecessary. Today, mullions are stylistically used but no longer needed for their functionality.

Grilles Represent Modern-Day Muntins and Mullions

Few window companies today distinguish between muntins, mullions, and stiles because few consumers understand the terms. The distinction between the terms hardly matters and confuses the window-buying process. Some companies may call them "muntins," or even more frequently, all of those terms fall under the more practical, universal term "grille."

Window manufacturer Pella calls all these elements a "grille," whether muntins, grids, windowpane dividers, or mullions. This is a sufficient one-size-fits-all word for a component without a structural function. Today, a mullion or grille is just there for aesthetic appeal.

The illusion of muntins is created by sandwiching thin strips of aluminum or plastic between double-paned glass or affixing these grilles onto the outside of the glass. They are usually offered as an add-on for an extra charge.

Much like houses have design elements like shutters that cannot close or columns that support no weight, the same is said for window grilles. They only exist for cosmetic and visual interest. For some, grilles attached to the outside of windows are even considered a nuisance since they must be removed to clean the window thoroughly.

Should You Get Window Grilles?

When you buy a new or replacement window today and want a grille, it will most likely be a simulated one. Grilles should fit the style of the house, for example, homes with a traditional look or nod to the Victorian Age.

If you have paned windows and want to spread the look throughout the house, adding cosmetic grilles can make it look consistent. It's easy to retrofit grilles to non-paned windows.

You can also get more expensive between-glass window grilles, which are permanent. When a window unit is made of two (or even three) panes of sealed glass containing argon or krypton gas, it's not a quick fix to open it up if you later decide you don't like the grilles.

You can buy windows that have clip-on window grilles. The grilles remain on the house's interior and can be removed for cleaning. Grilles sized for specific windows are available from the window manufacturer. Retrofit window grille kits are also available to allow you to build your grilles from separate, adjustable stiles and connectors. Kits range from $30 to $40 per window.