Lemma 29.20.5. Let $f : X \to S$ be a morphism of schemes. Let $x \in X$ be a point. Set $s = f(x)$. If $f$ is quasi-finite at $x$, then the residue field extension $\kappa (x)/\kappa (s)$ is finite.

**Proof.**
This is clear from Algebra, Definition 10.122.3.
$\square$

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