Mullions may be made of any material, but wood and aluminum are most common, although stone is also used between windows. Mullions are vertical elements and are often confused with transoms, which lie horizontally. The word is also confused with the "muntin" (or "glazing bar" in the UK) which is the precise word for the very small strips of wood or metal that divide a sash into smaller glass "panes" or "lights".
A mullion acts as a structural member, and it carries the dead load of the weight above the opening and the wind load acting on the window unit back to the building structure. The term is also properly applied to very large and deep structural members in many curtain wall systems.
When a very large glazed area was desired before the middle of the nineteenth century, such as in the large windows seen in gothic churches or Elizabethan palaces, the openings necessarily required division into a framework of mullions and transoms, often of stone. It was further necessary for each glazed panel, sash or casement to be further subdivided by muntins or lead cames because large panes of glass were reserved primarily for use as mirrors, being far too costly to use for glazing windows or doors.
In traditional designs today, mullions and transoms are normally used in combination with divided-light windows and doors when glazing porches or other large areas.
In cabinetry, the term "mullion" refers to any vertical member on a cabinet face that separates adjacent elements, usually doors or drawers. This same element is also called a "mid-stile."