Lemma 107.9.4. There is a decomposition into open and closed substacks

where each $\mathcal{C}\! \mathit{urves}_ g$ is characterized as follows:

given a family of curves $f : X \to S$ the following are equivalent

the classifying morphism $S \to \mathcal{C}\! \mathit{urves}$ factors through $\mathcal{C}\! \mathit{urves}_ g$,

$f_*\mathcal{O}_ X = \mathcal{O}_ S$, this holds after arbitrary base change, the fibres of $f$ have dimension $1$, and $R^1f_*\mathcal{O}_ X$ is a locally free $\mathcal{O}_ S$-module of rank $g$,

given a scheme $X$ proper over a field $k$ with $\dim (X) \leq 1$ the following are equivalent

the classifying morphism $\mathop{\mathrm{Spec}}(k) \to \mathcal{C}\! \mathit{urves}$ factors through $\mathcal{C}\! \mathit{urves}_ g$,

$\dim (X) = 1$, $k = H^0(X, \mathcal{O}_ X)$, and the genus of $X$ is $g$.

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