In the early third century B.C.E., Euclid gathered the existing knowledge about
geometry and combined it with his new work to form a comprehensive textbook of
geometry. He began with definitions and postulates (the fifth of which is the parallel
postulate) and includes many theorems and propositions. The Elements, consisting
of 13 books, is the oldest geometry book that has survived to modernity. This is
partly because the Elements was comprehensive for the time, so few other geometry
texts were needed or used. Consitent with his interest in spreading knowledge
through teaching, Euclid also founded and taught at a school in Alexandria.