Bibliotheca Himalayica, 1995. In calligraphy and typography entasis refers to the thinning in the waist of a stroke, brought about either by a slight heightening of the pen or brush angle and/or by an increase in the speed of the stroke as it goes into a straight. Visually it is reminiscent of the structure of a bone and a stroke is seen as stronger for it. Entasis in a stroke is intended to counter the illusion a stroke with perfectly straight sides has of bulging slightly. Conjecture about Greek columns The early Classical builders did not leave an explanation of their reasons for using entasis in their columns. Extensive conjecture about the purpose of its application by them exists. An early view, often articulated and still widespread, espoused by Hero of Alexandria, is that entasis corrects the optical illusion of concavity in the columns that the fallible human eye would create if the correction were not made. This view, however, does not explain the case of one well-known example, at Paestum on the western coast of Italy, where the entasis is so pronounced that it creates an obvious curvature, not an illusion of straightness. Some descriptions of entasis, state simply, that the technique was an enhancement applied to the more primitive conical columns to make them appear more substantial. Other descriptions argue that the technique emphasizes the substantiality of, not the columns, but rather, of some other part or of the building while being viewed as a whole. Yale architectural historian Vincent Scully argues that entasis emphasizes the weight of the roof of a building by making the building columns appear to bulge under the pressure distributed among them. Danish architect Steen Eiler Rasmussen believed that the effect represented strength by imitating the swelling of a strained muscle, a theory that accords well with the etymology of the word, from the Greek meaning "to strain". It also has been argued that a "stunted cycloid" column that bulges in the middle is stronger structurally than is a column whose diameter changes according to a linear progression, therefore, having a sound engineering purpose. Because their discussion of the application of the principle has never been discovered, it is unknown, however, whether the early Greeks knew this. If the column originally was a cultural reference to, or analogous to, the palm tree, the "bulge" is an accurate representation of the palm tree trunk.