In architecture, the column is the upright, cylindrical, load-bearing component that stands free of the walls and supports the entablature and roof of a building. It is distinguished from a pillar which may be square, oval, or rectangular by always being round. Its three sections are the capital, the shaft, and the base. The three distinct types of columns in classical Greek architecture are the Doric, Ionic, and Corinthian. Columns in a row constitute a colonnade.