What is a bay window?

A bay window is characterized by its projection outward from the main walls of a building, forming a recess in the room inside. The defining feature of a bay window is that it typically consists of at least three panels or sides, allowing for an expansive view and increased natural light from multiple angles. This design not only enhances the aesthetic appeal of a room but also creates a sense of spaciousness.

In contrast, the other options do not accurately represent what a bay window is. A vertical window on a roof would refer to a different architectural feature, possibly a dormer. A window without a frame does not provide the structural integrity or aesthetic value found in a bay window. Similarly, a flat window on a wall suggests a standard opening rather than the dynamic structure and profile of a bay window. Therefore, the option describing a window that allows light from three sides captures the essence of a bay window perfectly.