A Mullion is a vertical member that divides a wide opening. A mullion could be a structural element supporting lintel above and adding to the lateral stiffness to the window plane. Otherwise, mullions are functional and decorative components. Mullions provide side support to the glazing panels, or frames that hold the shutter or sash. Mullions as a side member or jamb are necessary for side hung casement windows, but can be avoided in friction hinge system.
A set of mullions around a central glazing pane creates side lites. One of the most famous side lites was created after Palladio. Mullions are straight members, but in gothic architecture the upper sections of the mullions branch out as foliage. Mullions are fluted with one profile through their length, but in post Industrial Revolution period mullions of cast steel with a floral configuration were used. Here mullions’ vertical nature was diffused and coalesced with transoms, which are nominally horizontal elements. In Rose windows the radial ribs or dividers have no distinct mullion or transom definition.
Mullions are important elements in stained glass windows. Mullions divide the mythological stories into sections, or a large story board is created by ignoring the intermediate presence of the mullions. This was also achieved by using mullions of very thin sections and by placing the stained glass on the inner edge of mullions’ face. This arrangement enhanced the divisions of the window architecturally on exterior face but created a large continuous or smooth plane on the interior face. Mullions of very thin size used to join small pieces of glass into a glazing pane and so with no structural role are called Muntins or glazing bars.
Mullions are made of materials such as wood, stone, steel, aluminium or polymers. Stone and wood mullions without glass panes are placed over openings to create a partially open space and for creating a sense of security, privacy and an architectural pattern.
In case of curtain wall system, a mullion is connected to floors, and transmits stresses during earthquake and other structural movements. Curtain wall mullions are highlighted on the exterior face as sticks or concealed within the window system by additional layer of internal glazing or panelling. Vertical elements of panelling and partition system, if add to the lateral stiffness are also called mullions. Vertical sticks or mullions allow unitization of window and grill system. Window panes with surrounding framing are assembled in a factory and inserted between the mullions on the site. Such a system also allows easy replacement of parts.