Mullion/muntin: A mullion is a heavy vertical or horizontal member between adjoining window units. Muntins are the narrow strips of wood that divide the individual panes of glass in a traditional sash.Click to see full answer. Subsequently, one may also ask, what is a horizontal mullion?A mullion is a vertical element that forms a division between units of a window or screen, or is used decoratively. Horizontal elements separating the head of a door from a window above are both a head jamb and horizontal mullion and are called “transoms”.Secondly, what is a door mullion? A mullion is a type of cabinet door that features glass panels rather than solid wood. The glass panels have dividing bars similar to those found in window panes. Similarly, what is a window Muntin? A muntin (US), 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 divide a single window sash or casement into a grid system of small panes of glass, called “lights” or “lites”.How thick are window mullions?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.