In architecture, entasis is the application of a convex curve to a surface for aesthetic purposes. Its best-known use is in certain orders of Classical columns that curve slightly as their diameter is decreased from the bottom upward. It also may serve an engineering function regarding strength. Etymology The word we apply to the design principle is used by the Roman architectural historian Vitruvius, and derives from the Greek word έντείνω (enteino), "to stretch or strain tight". Creating the illusion of greater strength or perception of height may have been an objective in the application of entasis. Examples Examples of this design principle may be found in cultures throughout the world, from ancient times to contemporary architecture. The first use of entasis is probably in the Later Temple of Aphaia at Aigina, in the 490s B.C. It also may be observed among Classical period Greek column designs, for example, in the Doric order temples in Segesta, Selinus, Agrigento, and Paestum. It was used less frequently in Hellenistic and Roman period architecture. The Roman temples built during these periods were higher than those of the Greeks, with longer and thinner columns. Chinese carpenters of the Song Dynasty followed designs in the AD 1103 Yingzao Fashi (Treatise on Architectural Methods or State Building Standards) that specified straight columns or those with an entasis on the upper third of the shaft. Noted architects, such as the Renaissance master Andrea Palladio, also used entasis in the designs of their buildings. Entasis was often a feature of Inca walls and double-jamb doorways, where they also act to counteract the optical illusion that would make the doorway appear narrower in the middle of its slope, than it really would be. It also may be seen in the sloping or battered walls of some Tibetan monastery and fortress architecture, as well as that of Bhutan. The lower parts of these walls, approximately one third, have a slight inward curve, but the higher parts are straight. If one builds a whole wall as a straight, sloping surface, it appears to bulge outward. An example in Bhutan is the Dobji dzong. When some collapsed walls of the Punakha dzong were rebuilt, around 1996, this wisdom about optical perceptions appears to have been overlooked or unknown to the restorers and, the rebuilt walls, being straight, appear to bulge, according to Chris Butters, the author of The Treasure Revealer of Bhutan,