Restriction of Symmetric Relation is Symmetric

Theorem
Let $S$ be a set.

Let $\mathcal R \subseteq S \times S$ be a symmetric relation on $S$.

Let $T \subseteq S$ be a subset of $S$.

Let $\mathcal R \restriction_T \, \subseteq T \times T$ be the restriction of $\mathcal R$ to $T$.

Then $\mathcal R \restriction_T$ is a symmetric relation on $T$.

Proof
Suppose $\mathcal R$ is symmetric on $S$.

Then:

and so $\mathcal R \restriction_T$ is symmetric on $T$.

Also see

 * Properties of Restriction of Relation‎ for other similar properties of the restriction of a relation.