Lemma 66.4.9. Let S be a scheme. Let X, Y be algebraic spaces over S. Let X' \subset X be an open subspace. Let f : Y \to X be a morphism of algebraic spaces over S. Then f factors through X' if and only if |f| : |Y| \to |X| factors through |X'| \subset |X|.
Proof. By Spaces, Lemma 65.12.3 we see that Y' = Y \times _ X X' \to Y is an open immersion. If |f|(|Y|) \subset |X'|, then clearly |Y'| = |Y|. Hence Y' = Y by Lemma 66.4.8. \square
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