The Stacks project

Lemma 58.31.1. Let $X' \to X$ be a morphism of locally Noetherian schemes. Let $U \subset X$ be a dense open. Assume

1. $U' = f^{-1}(U)$ is dense open in $X'$,

2. for every prime divisor $Z \subset X$ with $Z \cap U = \emptyset$ the local ring $\mathcal{O}_{X, \xi }$ of $X$ at the generic point $\xi$ of $Z$ is a discrete valuation ring,

3. for every prime divisor $Z' \subset X'$ with $Z' \cap U' = \emptyset$ the local ring $\mathcal{O}_{X', \xi '}$ of $X'$ at the generic point $\xi '$ of $Z'$ is a discrete valuation ring,

4. if $\xi ' \in X'$ is as in (3), then $\xi = f(\xi ')$ is as in (2).

Then if $f : Y \to U$ is finite étale and $Y$ is unramified, resp. tamely ramified over $X$ in codimension $1$, then $Y' = Y \times _ X X' \to U'$ is finite étale and $Y'$ is unramified, resp. tamely ramified over $X'$ in codimension $1$.