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A muntin is a strip of material that holds individual panes of glass in a larger frame, like in a window, door, or piece of furniture. Although still used in construction today, these were traditionally — in the 18th and 19th centuries — used to make windows with small panes of glass. Typically, these sheets were glazed into a wooden or metal grillwork, which was known as a muntin. It was a key component, and held the panes in place in the larger frame of the window, known as a sash.
The panes of glass are held in place, in a muntin, by way of a groove routed into the its edge. This groove is known as a fillet. The pane is fitted into the fillet, and can then held in place by a few different methods. The pane can be glazed, using putty and glazing points to hold it. Or, small strips of wood or metal can be fit into the fillet to hold the pane in place.
Muntins that actually separate individual panes of glass are also known as true divided lights. Other terms for muntins are muntin bars, sash bars, and glazing bars. Sometimes they are referred to simply as grilles.
Windows manufactured today no longer need to be made from small panes of glass. Often, however, the division in larger windows that muntins provide is seen as an important decorative element, enhancing the design of a building. For this reason, false muntins are sometimes used.
A false muntin can be a decorative element sandwiched between insulated glass panes, giving the effect of divided lights. It can also be attached to the outside of a large window — overlaying it and giving the illusion of the window being made of smaller panes. Sometimes both techniques are used to give the illusion of a true divided light.
As the manufacture of large panes of glass became easier, many historic buildings that had traditionally contained windows with true divided lights replaced those with larger panes in a single sash. Also, as many of these historic buildings are restored, the windows with muntins are often redone to create larger-paned windows that would have historically been divided. This adds to the authenticity of a building’s restoration.
Traditionally, wood or various metals were used to make early versions of muntins. Lead was sometimes used because of its malleability. Today, especially when false muntins are used for decorative effect, they are generally made of plastic or aluminum.