This subsection contains the definitions 1 to 4 from Book 10 of Euclid's “Elements”.

- Definition: Def. 10.01: Magnitudes Commensurable and Incommensurable in Length
- Definition: 10.02: Magnitudes Commensurable and Incommensurable in Square
- Definition: Def. 10.03: Rational and Irrational Magnitudes
- Definition: Def. 10.04: Rational and Irrational Magnitudes in Square