Why did Euclid state that all right angles are equal?

Euclid did not provide a specific reason for stating that all right angles are equal because he categorized it as an unprovable but intuitive principle, known as a postulate or axiom.1

In his foundational work, the Elements, Euclid sought to build a rigorous system of geometry starting from basic definitions and self-evident truths. He proposed five postulates to serve as the basis for all subsequent logical deductions. The fourth postulate, "all right angles are equal," was included alongside others—such as the ability to join two points with a straight line—to establish the necessary framework for his proofs. These axioms were intended to be intuitive starting points that did not require further proof, allowing him to derive more complex theorems throughout his work.1