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 going back a few hundred years to England, where both terms originated.
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.
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.
But 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.
Mullions are different from muntins, though they both act as supporting devices. Generally, mullions are the single vertical bars that separate two sides of a single window.
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.
Muntin vs. Mullion
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.
Grilles: Alternative to Muntin and Mullion Terminology
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.
About Misspellings: Mutton and Muntins
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.