The mullion is framing element which divides adjacent window or door units. Mullions can be processed of any poop, however wood and metal are most common, although stone is also utilized around windows. Mullions come virtually all usually vertical or even horizontal, for instance diagonal, although additional complex arrangements were popular in a period of the Tudor era.

It used to be that, mullions were necessary because it was non conceivable to make sufficiently big panes of glass. In todays world, it is primarily utilized for cosmetic purposes. Within double-glazed windows, a grid resembling mullions is occasionally sandwiched between a panes when decoration.