Mullion and Muntin

In a glass-paned door or window, muntins are the thin strips dividing the individual panes of glass within one sash or frame, while mullions are the larger structural members dividing separate window or door units from each other -- the distinction is scale and structural role, not appearance. Both terms show up in furniture and cabinetry wherever glass-front doors are built with a divided-light look, whether the divisions are structural (actually holding separate panes) or, on modern doors, a purely decorative applied grid over one continuous pane of glass.