The optical refinement that gives the Parthenon’s columns a slight swelling is entasis.
Entasis is a subtle convex curve in a column shaft. Instead of tapering upward in a perfectly straight line, the shaft narrows gently, creating a barely perceptible fullness—usually below the midpoint. Ancient architects used this refinement to make columns appear straighter, stronger, and more graceful when viewed from the ground.
The Parthenon’s entasis is especially restrained. Measurements indicate that its effect follows an approximate ratio of 1:550 to 1:600, so the swelling is difficult to detect without careful study. It forms part of a larger system of optical corrections that also includes the upward curve of the stylobate, inward-leaning columns, and slight adjustments to the corner columns.
A common mix-up is confusing entasis with the stylobate. The stylobate is the platform supporting the columns; entasis describes the curved profile of the columns themselves. The term derives from the Greek for “to stretch” or “to strain tight” and was discussed by the Roman architect Vitruvius.