Entasis

In architecture, entasis is a design technique used to counteract a certain optical illusion. When a column or other structure is designed with parallel sides, the sides appear to have a slight inward curve. Entasis is the inclusion of a slight outward curve in the sides – making them not parallel – to counteract this optical effect. The goal, somewhat paradoxically, is to give the appearance of straight lines through the use of curves and optical illusion.

It was likely first used in the contruction of the Egyptian pyramids, but can also be seen in Greek column designs. Noted architects like the Renaissance master Andrea Palladio used it in their buildings. More recently, Rolls-Royce cars all made use of it in their radiator grills.