mullion (mŭlˈyən) [key], in architecture, a slender, upright intermediate member that subdivides an opening, as a division between panes of a window or between adjacent windows. Although the mullion occurs in some form in nearly all architectural styles, it is perhaps most characteristic of the elaborate Gothic systems of stone tracery.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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