Double-hung vinyl replacement window with a decorative grille resembling mullions sandwiched between the panes of the insulated glass.

A mullion is framing element which divides adjacent window or door units. Mullions may be made of any material, but wood and metal are most common, although stone is also used in windows. Mullions are most commonly vertical or horizontal, sometimes diagonal, although more complex arrangements were popular during the Tudor era.

In the past, mullions were necessary because it was not possible to produce sufficiently large panes of glass. Today, they are mainly used for decorative purposes. In double-glazed windows, a grid resembling mullions is sometimes sandwiched between the panes as decoration.

See also

• came
• stained glass

Mullion is also the name of a village in Cornwall off the Lizard.

Mullion is also a city with over one million mullets (hair cut)

See also

• coupe longueuil