**Theorem (Shoenfield's Absoluteness Theorem).** Every $\Sigma_2^1(a)$ relation and every $\Pi_2^1(a)$ relation is absolute for all inner models $M$ of $\mathrm{ZF+DC}$ such that $a \in M$. In particular, $\boldsymbol{\Sigma}_2^1$ and $\boldsymbol{\Pi}_2^1$ relations are absolute for $L$.