A cupola is a decorative, tiny, protruding tower at the top of a building's roof, which is often square, round, or octagonal in shape. A window opening in the roof, capped by a front gable or shed roof, The overhanging edge of the roof that extends beyond the outside walls, sometimes with exposed rafters. The term can also be used to describe the structure that houses the opening. Cupolas are found on many buildings across the world, especially churches and smaller residential buildings. They were commonly used as observatories by scientists and scholars, and today some museums include small cupolas in their roofs as well. Cupolas are also found on larger structures such as factories and schools where they provide an open area for students to study or teachers to teach classes from. There are two main types of cupolas: those with domes and those with shells. Shell cupolas have a flat or slightly sloped cover instead of a dome. They tend to be cheaper to build and less weather-resistant than domed cupolas. However, they look more modern and can be used as architectural features rather than just functional ones. Dome cupolas are shown higher up on this page because they are more prestigious and desirable than shell cupolas. Dome cupolas require a much steeper roof slope to work effectively, so they are not practical for all buildings. However, they make up most of the cupolas found on churches and other religious buildings throughout history.