thumb|upright|diagram of a corinthian column showing a visible entasis bulge at "d" 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 upwards. in the hellenistic period some columns with entasis are cylindrical in their lower parts. some roman columns are "cigar-shaped", with the widest point some distance above the foot. the word derives from the greek word ÎµÎ½ÏÎµÎ¯Î½Ï (enteino), "to stretch or strain tight". ==examples== probably the first use of entasis was in the construction of the egyptian pyramids, but it can also be observed in classical period greek column designs, for example in the doric-order temples in segesta, selinus, agrigento and paestum. it was used less 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 the designs in the a.d. 1103 yingzao fashi (treatise on architectural methods or state building standards) which specified straight columnns or with an entasis on the upper third of the shaft.liang, sicheng, and wilma fairbank, ed.. a pictorial history of chinese architecture: a study of the development of its structural system and the evolution of its types. cambridge, mass.: mit press, 1984. 17. noted architects such as the renaissance master andrea palladio also used it in 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.protzen, jean-pierre. "inca architecture" in the inca world, laura laurencich minelli (ed). university of oklahoma press, 2000, pp. 196-197. ==purpose==   the early classical builders did not leave an explanation of their reasons for using entasis, and there are several differing opinions as to its purpose: an early-articulated and still widespread view, espoused by hero of alexandria, is that entasis corrects the optical illusion of concavity in the columns which the fallible human eye would create if a correction were not made.hero of alexandria, horoi ton geometrias onomaton, 135, 14: "thus, since a cylindrical column would, when looked at, seem irregularly narrower in the middle, he make
Columns_and_entablature Optical_illusions