Why are the Parthenon columns not straight?

"Despite appearances, there are few perfectly straight lines or right angles in the Parthenon. The observer sees the eight columns of the façade as a perfectly regular array, but this is achieved by deliberately introducing subtle distortions called “optical refinements”"

"" The internet

The columns themselves are not straight along their vertical axes, but swell in their middles. This phenomenon, called “entasis” intended to counteract another optical effect in which columns with straight sides appear to the eye to be slenderer in their middles.

Explanation:

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