Lemma 49.7.3. Let $f : Y \to X$ be a morphism of schemes which is locally of finite type. Let $R \subset Y$ be the closed subscheme defined by the Kähler different. Then $R \subset Y$ is exactly the set of points where $f$ is not unramified.

Proof. This is a copy of Divisors, Lemma 31.10.2. $\square$

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