A mullion is a vertical bar the separates a glass window into segments, or panes. They are used to break up a space on a window. Often times, you will find mullion windows used in gothic style homes and building structures. This style strictly correlates with the romantic, gothic style used in many literary references.
How has Mullion Changed?
Throughout the years, mullion has really changed in its purpose. When the use of this material first appeared on the scene, it was used mainly for two reasons. These reasons were to help support the structure, mainly the weight of the structure around them, and to be used as fillers. In the time that Mullion found its use, glass was a very expensive material to be had and used. The bar was used to separate and take up space where less glass was needed so smaller panes could be used.
In more recent times, you will see mullion made from a variety of material such as stone, wood, metal, and plaster. They do not act as structural components but are meant more for a decorative appeal.