A mullion is a vertical element that forms a division between units of a window, door, or screen, or is used decoratively. When dividing adjacent window units, its primary purpose is to provide structural support to an arch or lintel above the window opening. Its secondary purpose may be as a rigid support to the glazing of the window. When used to support glazing, they are teamed with horizontal supporting elements called "transoms".
In the commercial door industry, the term is also applied to a piece of hardware that divides the opening of a pair of doors.
A window shutter is a solid and stable window covering usually consisting of a frame of vertical stiles and horizontal rails (top, centre and bottom). Set within this frame can be louvers (both operable or fixed, horizontal or vertical), solid panels, fabric, glass and most any other item that can be mounted within a frame. Shutters may be employed for a variety of reasons, including controlling the amount of sunlight that enters a room, to provide privacy, security, to protect against weather or unwanted intrusion or damage and to enhance the aesthetics of a building. Depending on the application, and the construction of the window frame, shutters can be mounted to fit within the opening or to overlap the opening. The term window shutter includes both interior shutters, used on the inside of a house or building, and exterior shutters, used on the outside of a structure. On some styles of buildings it is common to have shutters to cover the doors as well as the windows.