Symmetric Preordering is Equivalence Relation

Theorem
Let $\mathcal R \subseteq S \times S$ be a preordering on a set $S$.

Let $\mathcal R$ also be symmetric.

Then $\mathcal R$ is an equivalence relation on $S$.

Proof
By definition, a preordering on $S$ is a relation on $S$ which is:
 * $(1): \quad$ reflexive

and:
 * $(2): \quad$ transitive.

Thus $\mathcal R$ is a relation on $S$ which is reflexive, transitive and symmetric.

Thus by definition $\mathcal R$ is an equivalence relation on $S$.