Usually as "muntin bars." Thin strips of wood between the individual panes of glass in a "true-divided light" window or French door. May be applied externally to a larger single pane to simulate a true divided light.
http://www.fenplast.com/en/window-door-grilles.aspx
Do not confuse with mullion, which is a structural element.
http://www.fenplast.com/en/window-door-grilles.aspx
Do not confuse with mullion, which is a structural element.