Mullions can be decorative or functional, fixed or removable, and can be found in all styles of Western architecture. So what are they” Vertical elements that form a division between units of a window, door, or screen. In layman’s terms, they’re the thingies dividing the glass panes. More specifically, mullions can divide glass within a door, between a door and a sidelight, within only glazed areas, or in panels. In other words, in this world there are millions of mullions.