The Greek mathematician Euclid (c. 325-c. 270 B.C.E.), contributed to the development of arithmetic and the geometric theory of quadratic equations. Although little is known about his life— except that he taught in Alexandria, Egypt—his greatest contributions to geometry are well understood. The elementary geometry many of us learn in high school is still largely based on Euclid. His thirteen books of geometry and other mathematics, titled *Elements* (or *Stoicheion* in Greek), were classics of his day. The first six volumes offer elementary plane geometry; the other books present the theory of numbers, certain problems in arithmetic (on a geometric basis), and solid geometry. He also defines basic terms such as point and line, certain related axioms and postulates, and then a number of statements logically deduced from the definitions, axioms, and postulates. (For more information on axiom and postulates, see “Foundations of Mathematics”; for more information about Euclid, see “Geometry and Trigonometry.”)