Axiom: 1.4: Equality of all Right Angles
(Postulate 4 from Book 1 of Euclid's “Elements”)
And that all right angles are equal to one another.
Modern Formulation
All right angles are congruent.
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References
Adapted from (subject to copyright, with kind permission)
- Fitzpatrick, Richard: Euclid's "Elements of Geometry"
