Terms of the Trade: What Is a Mullion? Terms of the Trade: What Is a Mullion?
You've probably seen this decorative or architectural element a lot without even realizing it. A mullion is a vertical element that divides a window, screen, or door and is often used for structural support or for window glazing. The use of mullions dates backs thousands of years to the 10th century and became popular in Europe within Romanesque and Gothic architecture, particularly in churches.