Lemma 75.23.6. Let $S$ be a scheme. Let $f : X \to Y$ and $Y \to Z$ be a morphism of algebraic spaces over $S$. Assume
$X$ is locally of finite presentation over $Z$,
$X$ is flat over $Z$, and
$Y$ is locally of finite type over $Z$.
Then the set
is open in $|X|$ and its formation commutes with arbitrary base change $Z' \to Z$.