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Ancient architects sometimes deliberately adjusted supposedly straight architectural elements.
Columns could be made slightly thicker toward the middle, and horizontal lines could receive subtle curvature.
Why?
Perfectly straight or perfectly parallel structures can sometimes look slightly distorted to human eyes when viewed from a distance.
Architects could compensate for these visual effects by making tiny physical adjustments.
The finished building could therefore appear more symmetrical and perfectly straight than it would if every measurement were mathematically uniform.
In other words, the architect sometimes had to make something technically imperfect to make it look perfect.
© Encyclopaedia Britannica