The first known usage of a single letter to denote the 6.28... constant was in Leonhard Euler 's 1727 Essay Explaining the Properties of Air, where it was denoted by the letter π. Euler would later use the letter π for the 3.14... constant in his 1736 Mechanica and 1748 Introductio in analysin infinitorum, though defined as half the circumference of a circle of radius 1—a unit circle —rather than the ratio of circumference to diameter. Elsewhere in Introductio in analysin infinitorum, Euler instead used the letter π for one-fourth of the circumference of a unit circle, or 1.57.... Eventually, π was standardized as being equal to 3.14..., and its usage became widespread.