How many books are in Euclid's Elements of Geometry?

The story behind the answer

Euclid’s Elements consists of 13 books. Books I–VI develop plane geometry, VII–IX cover number theory (including prime numbers and the Euclidean algorithm), Book X studies incommensurables (irrational magnitudes), and XI–XIII build 3D geometry, culminating in the construction and classification of the five Platonic solids. Some later editions include Books XIV and XV by other authors, which can cause confusion, but Euclid’s original work has 13.

Composed around 300 BCE in Alexandria, the Elements was the foundational math textbook for over two millennia, shaping the axiomatic method used throughout mathematics. Memory aid: think “6 + 3 + 1 + 3 = 13”—six books for plane geometry, three for numbers, one for irrationals, three for solids.