Angle comparisons are important for properties that will be used in later posts. Congruence Congruence in angles is a concept that is similar to equality of numbers. Here congruence is used with the symbol ($\equiv$) and this primitive notion satisfies the postulates: Reflective every angle is congruent to itself. $\widehat{ab} \equiv \widehat{ab}$ Symmetric If $\widehat{ab} \equiv \widehat{cd}$ then $\widehat{cd} \equiv \widehat{ab}$ Transitive If $\widehat{ab} \equiv \widehat{cd}$ and $\widehat{cd} \equiv \widehat{ef}$ then $\widehat{ab} \equiv \widehat{ef}$