A vertical wood piece separating combined window units is known as what?

The correct term for a vertical wood piece that separates combined window units is known as a mullion strip. A mullion serves a structural and aesthetic purpose by providing support between two window units, thereby allowing multiple windows to be installed together while maintaining a cohesive look.

In addition to the functional aspects, mullions can enhance the design of the window by breaking up the visual field, which can be particularly important in larger window installations or in architectural designs that call for a specific style.

Understanding the other building components helps clarify why the mullion is distinct. A header is typically used to support the weight of the structure above an opening, like a door or window but does not serve to separate units. A sill refers to the horizontal member at the bottom of a window frame, and a jamb pertains to the vertical sides of the window frame, providing an edge for the installation. These components have different roles and do not function as separators between multiple window units, making the mullion the correct answer in this context.