A mullion is a vertical element that forms a division between units of a window or screen, or is used decoratively. When dividing adjacent window units its primary purpose is a rigid support to the glazing of the window.

What is meant by mullioned?

/ˈmʌliənd/ [only before noun] (architecture) ​(of a window) having one or more solid vertical pieces of stone, wood or metal between different parts of the window. mullioned windows.

What is a mullioned window?

A mullion refers to the vertical piece of wood separating the panes of glass, not both the verticals and the horizontal stile pieces. Today, mullions are the vertical bars between the panes of glass in a window. As with muntins, their role is primarily decorative now.