The diameter of classical columns slightly decreases with height. Vitruvius
calls it “entasis” and cites aesthetic reasons for its use already in Greek
architecture. However, the exact geometrical shape of the entasis remained
unexplained. It was subject to wide speculation even in later times. The large
columns at the Porticus of the Pantheon in Rome are made with an entasis. This
article describes the exact measurements of their proportion and a method to
determine the geometry of the entasis. The Bern Digital Pantheon Model is used
as the data source.