What does MULL stand for?

Mullion

Commonly used as a brick cleaner after masonry work is completed. A vertial member or division between the panels of a window. A vertical divider in the frame between windows, doors, or other openings. 1. The vertical dividing bar of a rose or lancet window. 2. A vertical primary framing member that separates paired or multiple windows within a single opening.3.A slender upright dividing an opening, usually a window, into two or more sections.