Muntins in Windows: Definition & Style

Instructor: Sunday Moulton

Sunday recently earned a PhD in Anthropology and has taught college courses in Anthropology, English, and high school ACT/SAT Prep.

This lesson introduces muntins used in windows, how their use came about, the difference between muntins and mullions, and a number of styles created by different placement of muntins in a window.

Practical Elegance of Muntin Windows

Have you ever noticed the windows in animated films like Beauty and the Beast, Snow White, or Sleeping Beauty, and the elegance of the glass separated into smaller frames by wooden or metal bars called muntins? Do you ever look around at buildings in your community, usually older constructions, with similar windows, sometimes with the panes of glass separated in interesting or intricate patterns? This design feature of windows serves both a functional and an aesthetic purpose. While the aesthetic is obvious, especially to window enthusiasts, the function of this style is rooted in the historic manufacturing of glass.

While glass is mass-produced these days, it used to be a challenging and expensive product to manufacture. Often, glass windows appeared only in the dwellings of aristocrats or in wealthy churches. Yet even then, large sheets of glass cost exorbitant amounts, and replacing a cracked window of that size was a nightmare. As a solution to both the production and replacement cost problems, windows were often constructed of smaller panes of glass, separated and held in place by wooden or metal bars set inside the larger window frame. Smaller panes cost lest to manufacture. As size increased, so did the chance of defects or accidents, and smaller panes could still be cut from the remains of a large, broken sheet. Replacement and repair costs decreased as accidents usually broke only a few panes instead of the entire window.

Muntin Versus Mullion

Now, before we go any further into the styles of such windows, we should pause to address one of the most common confusions in architecture and design, the muntin versus the mullion. So great is this confusion that even websites and blogs addressing this problem can sometimes mix them up. Here are the two definitions.

Muntin: small bar or strip, usually wooden but occasionally metal or vinyl, placed inside a window's frame to hold smaller panes of glass

Mullion: larger, vertical bar or post, usually made of wood, used to separate two or more window casements that occupy a larger window opening

Neither of these 17th-century words are in common use today, so the confusion is understandable. However, it is important to remember that muntins are smaller and inside the frame while mullions separate entire window casements. An easy way to remember is by looking at the words. Muntin has a lowercase 't' which resembles the intersection of a horizontal and a vertical muntin in a window. The word mullion, however, has a double, lowercase 'L' in the middle, resembling vertical posts holding together window casements set side by side.

Most Common Styles

So now that we know a little history of muntin windows and have clarity on the muntin-versus-mullion issue, let us look at a number of styles developed to create interesting patterns from the grid work of muntins. While a creative carpenter should be able to fashion a window with any of an infinite number of variations, the three most common styles are Colonial, Prairie, and Craftsman.

Colonial: Most frequently used in double-hung windows (two windows hung together in a track to allow opening and closing by sliding them up or down), muntins divide each window into a number of equally shaped and sized panes. Usually holding six panes in each window with two vertical and one horizontal muntin, variations of the colonial style include 4-pane, 8-pane, or 9-pane windows. The illustration below is an example of a double-hung, Colonial window.

Prairie: This style divides the window into nine panes with two horizontal and two vertical muntins placed a short distance from the window's outer frame. This creates a large center pane, four small corner panes, and four long, narrow panes on each side.

Craftsman: The simplest style of muntin window, Craftsman divides the window into two equal panes with a centrally-located, vertical muntin. The illustration below is an example of a double-hung Craftsman window.

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