What is the vertical piece between two windows called?

The vertical piece between two windows is referred to as a mullion. This architectural feature serves not only a functional purpose by providing structural support and stability to the window assembly but also contributes to the aesthetic appearance of a building's facade. Mullions can create a sense of division between different window units or panels, which can enhance the visual interest of the design.

Mullions are typically constructed from various materials such as wood, metal, or composite materials and can vary in size and style depending on the architectural requirements and intent of the design. They are particularly common in traditional and modern architectural styles, where creating a rhythm or pattern in window design is important.

Understanding the role of a mullion is crucial in architecture as it impacts both the functionality of windows in terms of support and thermal performance, as well as the overall visual composition of a building's exterior.