Definition 35.34.11. Let $S$ be a scheme. Let $\{ X_ i \to S\} $ be a family of morphisms with target $S$.

Given a scheme $U$ over $S$ we have a

*canonical descent datum*on the family of schemes $X_ i \times _ S U$ by pulling back the trivial descent datum for $U$ relative to $\{ \text{id} : S \to S\} $. We denote this descent datum $(X_ i \times _ S U, can)$.A descent datum $(V_ i, \varphi _{ij})$ relative to $\{ X_ i \to S\} $ is called

*effective*if there exists a scheme $U$ over $S$ such that $(V_ i, \varphi _{ij})$ is isomorphic to $(X_ i \times _ S U, can)$.

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