mullion
noun
a vertical member, as of stone or wood, between the lights of a window, the panels in wainscoting, or the like.
A verb form that is used in progressive verb constructions and may also function as a modifier or a gerund.
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They gave no sign of anybody inside, and, indeed, the mullioning of time had so dimmed them that, perhaps, the outside world could hardly be seen from within.
From The Black Colonel by James Milne