Chapter 1: Euclid’s Elements, Props 1-28 of Book 1
( \newcommand{\kernel}{\mathrm{null}\,}\)
Thumbnail: P. Oxy. 29, one of the oldest surviving fragments of Euclid's Elements, a textbook used for millennia to teach proof-writing techniques. The diagram accompanies Book II, Proposition 5. (Public Domain). Text from Oscar Levin's Discrete Mathematics text (CC BY-SA).