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)

  1. Fitzpatrick, Richard: Euclid's "Elements of Geometry"