Muntin

A muntin (US), the muntin bar, glazing bar (UK), or sash bar is a strip of wood or metal separating and holding panes of glass in a window. Muntins can be found in doors, windows, and furniture. Muntins divide a single window sash or casement into a grid system of small panes of glass, called "lights" or "lites."

Windows with "true divided lite (TDL)" make use of thin muntins, typically 1/2" to 7/8" wide in residential windows, positioned between individual panes of glass. In wooden windows, a fillet is cut into the outer edge of the muntin to "stop" the pane of glass in the opening, and a putty or glazing bead is then used to hold the glass in place. The inner sides of wooden muntins are typically milled to traditional profiles. In the U.S., the thickness of window muntins has varied historically, ranging from very slim muntins in 19th-century Greek revival buildings to thick muntins in 17th and early 18th century buildings.

Until the middle of the 19th century, it was economically necessary to use smaller panes of glass, which were much more affordable to produce, and fabricate into a grid to make large windows and doors. Many considered the division of a window or glazed door into smaller panes to be more architecturally attractive than large panes. 

Muntins are often confused with "mullions" (which separate complete window units) and "astragals" (which close the gap between two leaves of a double door). Many companies use the term "grille" when referring to a decorative structure of aluminum grilles between the glass (GBG) or wood snap-on grilles (removable) for interior glass surface to make it look as if muntins were separating multiple panes of glass.