The architects of the Parthenon used a design technique known as entasis to make the columns appear straight from a distance. Entasis involved subtly curving the columns, so that they bulged slightly in the middle. This curvature counteracted the optical illusion known as the "concave effect," which makes straight lines look curved when viewed from a distance. By employing entasis, the architects ensured that the columns of the Parthenon appeared perfectly straight to the viewer, even when viewed from afar.
