The Greek mathematician and geometrician Euclid (c. 325-c. 270 B.C.E.) made some of the most significant improvements to geometry in his time. (For more about Euclid, see “History of Mathematics” and “Foundations of Mathematics.”) One contribution was his collection of 13 books on geometry and other mathematics, titled *Elements* (or *Stoicheion* in Greek). This work has been called the world’s most definitive text on geometry. The first six books offer elementary plane geometry, with sections on triangles, rectangles, circles, polygons, proportions, and similarities; the rest of the books present other mathematics of his day, including the theory of numbers (books 7 to 10), solid geometry, pyramids, and Platonic solids. These books were used for centuries in western Europe; in fact, the elementary geometry many students learn in high school today is still largely based on Euclid’s ideas on the subject.