Eudoxus of Cnidus
Pythagoras (c. 570–495 BC) was probably the source for most of books I and II, Hippocrates of Chios (c. 470–410 BC, not the better known Hippocrates of Kos) for book III, and Eudoxus of Cnidus (c. 408–355 BC) for book V, while books IV, VI, XI, and XII probably came from other Pythagorean or Athenian mathematicians. The "Elements" may have been based on an earlier textbook by Hippocrates of Chios, who also may have originated the use of letters to refer to figures.