Lemma 26.23.2. Let $j : X \to Y$ be a morphism of schemes. Then $j$ is a monomorphism if and only if the diagonal morphism $\Delta _{X/Y} : X \to X \times _ Y X$ is an isomorphism.

** A scheme morphism is a monomorphism iff its diagonal is an isomorphism. **

**Proof.**
This is true in any category with fibre products.
$\square$

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Comment #861 by Kestutis Cesnavicius on