Note:  Do not rely on this information. It is very old.

Mullion

Mullion, in Gothic architecture, a slender pier or vertical bar of stone separating the lights of windows, etc., different from the transom (q.v.), which is horizontal. They arose in the Early English style; as the group of two or more windows placed close together was formed into a single window containing two or more lights, the separating pieces of wall became mullions.