What architectural shape forms the ceiling of the Sistine Chapel?

The Sistine Chapel’s ceiling is a barrel vault.

A barrel vault is a continuous, tunnel-like curved ceiling generated by extending an arch along a length. In the chapel, this architectural form provided the broad curved surface that Michelangelo transformed into one of the world’s best-known painted ensembles.

Michelangelo’s ceiling decoration includes the central Genesis scenes, prophets and sibyls, ignudi, ancestors of Christ, and other figures. The vault’s curvature made the project especially demanding because the imagery had to be designed for viewers looking upward from the floor.

The ceiling is often casually called a dome because it is high and monumental, but it is not a dome. Its basic structural form is a barrel vault, with pendentive-like triangular areas around the windows that helped organize the painted program.