The Parthenon on the Athenian Acropolis is a Doric temple designed by the architects Iktinos and Kallikrates (under the oversight of the sculptor Pheidias). Its structure is post-and-lintel in its purest form: marble architraves on marble columns. The real lesson lies not in the form but in the form's deliberate imperfections.

The human eye often reads perfectly straight lines and right angles as slightly sagging or concave. To reverse this, the Parthenon's architects left almost no line of the building truly straight. The shafts of the columns swell gently at the middle (entasis), so the upward-tapering profile looks taut and alive rather than lifeless. The platform they stand on (the stylobate) describes a convex curve, rising several centimetres toward the centre. The corner columns are a little thicker than the rest and lean slightly toward their neighbours; the spacing tightens toward the edges. Extended upward, all the columns would lean inward to meet kilometres above in the sky.

None of these deviations is accidental; each compensates for the eye's natural distortion to make the building appear perfectly straight. The Parthenon thus gives architecture a lasting discipline: measured geometry and perceived geometry are different things, and design aims at the perceived.