A mullion is a vertical or horizontal structural element that divides a window or a door into smaller sections. It is commonly found in architectural designs, providing support and enhancing the aesthetic appeal of the opening. Mullions can be made from various materials, including wood, metal, or stone, and are often used in conjunction with transoms, which are horizontal bars. In addition to their structural role, mullions can also influence the amount of light entering a space and the overall style of a building. They are often seen in gothic architecture and modern designs, contributing to the character and functionality of windows and doors.