What term describes a small vertical or horizontal piece that holds a pane of glass in a window?

The term that describes a small vertical or horizontal piece that holds a pane of glass in a window is "muntin." Muntins are the thin strips of material, typically wood or metal, which separate individual panes of glass within a window frame. They are essential in traditional window designs where multiple small panes are used instead of a single large pane, providing structural support and contributing to the aesthetic appeal of the window.

The other terms in the list refer to different components of window design. For instance, a mullion is a larger vertical element that divides two or more windows or larger sections of glass rather than individual panes. The frame is the overall structure that holds the entire window assembly, and the sash refers to the movable part of the window that holds the glass panes and can be opened or closed. Thus, while each term is related to windows, "muntin" specifically pertains to the individual pieces that separate the panes of glass.