Lemma 37.16.6. Let $S$ be a scheme. Let $f : X \to Y$ be a morphism of schemes over $S$. Assume
$X$ is locally of finite presentation over $S$,
$X$ is flat over $S$, and
$Y$ is locally of finite type over $S$.
Then the set
is open in $X$ and its formation commutes with arbitrary base change.