Window Mullions vs. Window Muntins

Real or Simulated, These Elements Influence Windows' Appearance

Window Mullions

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Muntins and mullions are words that don't exactly roll off of your tongue. Yet it is possible that your modern home has muntins or mullions on its windows, whether in real or simulated form. Plus, because the two words sound alike, they often get confused.

Is a muntin the same thing as a mullion? What about those other words—stiles and grilles? How do they play into this? Unraveling the mystery means traveling back a few hundred years to England, where both terms originated.

Window Muntins

What Is a Window Muntin?

Muntin refers to the vertical dividers that separate glass panes in a window. Muntin applies only to the inner vertical pieces; the outer pieces that form the frame are stiles and rails.

Even though muntins today are most frequently associated with windows, they can mean any kind of vertical divider, whether for windows, wood panels, furniture, or doors.

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 that references, "English joiners [who] apply the term muntin to the intermediate upright bars of framing, and call the outside uprights styles." Styles is usually expressed today as stiles.

Early builders used muntins not for looks but because they were structurally necessary. Early buildings' outer walls could not carry the weight when large windows were placed in the walls. Muntins allowed for weight to be transmitted vertically.

This would especially have been the case with Westminster's vast amounts of fenestration: Its west window is close to 45 feet high.

Window Mullions

What Is a Window Mullion?

Generally, mullions are the single vertical bars that separate two sides of a single window. 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.

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 calls a mullion "A vertical bar between the panes of glass in a window." Prior to the Victorian Age and the advent of inexpensive plate glass, it was impossible or prohibitive to manufacture large sheets of glass. Giant expanses of windows were achieved by holding smaller panes of glass together by 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.

Post-War Years and Plate Glass

After World War II, mullions and muntins fell out of favor. With larger expanses of tempered plate glass being available on the residential market, small panes of glass separated by supports were no longer necessary. Muntins and mullions were a sign of old-fashioned building: ungainly. obstructive, unnecessary.

Windows with mullions would always remain as stylistic devices, but never again functionally.

Grilles: Alternative to Muntin and Mullion Terminology

In the truest sense, muntins are the vertical pieces of wood separating panes of glass in a classic multi-pane glass assembly. Mullions are the single vertical supports used in two-pane assemblies.

But few window companies today make a distinction between muntins, mullions, and stiles because few window buyers care about this. This is a fine distinction that rarely matters, and which only makes the window buying process more confusing.

As a consequence, if these terms are even used, the elements might all be called muntins. Even more frequently, all of those terms fall under the more useful, universal term grille.

Major window manufacturer Pella calls all of these elements a grille, whether muntins, grids, windowpane dividers, or mullions. This is a sufficient one-size-fits-all word for an element that no longer has a structural function. Today, a mullion or grille is just there for aesthetic appeal.

Window Grille Construction

One of the interesting aspects of the modern house is the way that old and often unnecessary elements are retained, even after their functional use is no longer needed. Shutters that cannot close and columns that support no weight are frequently found on house facades.

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

These types of window grilles have no functional purpose; they are only there for visual appeal. In fact, grilles attached to the outside of the window can be a nuisance since they have to be removed in order to clean the window.

Misspellings: Mutton and Mutin

Muntin sometimes gets its "n" dropped, becoming "mutin" or "mutton." Both are incorrect. 

"Mutin" is not a word. "Mutton" is a word but it has nothing to do with windows or buildings at all; it refers to sheep meat.

Article Sources
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  1. Mullion. Oxford English Dictionary.