Lemma 105.5.9. Suppose given a Cartesian square of morphisms of locally Noetherian stacks

$\xymatrix{ \mathcal{T}' \ar[d]\ar[r] & \mathcal{T} \ar[d] \\ \mathcal{X}' \ar[r] & \mathcal{X} }$

in which the vertical morphisms are locally of finite type. If $t' \in |\mathcal{T}'|$, with images $t$, $x'$, and $x$ in $|\mathcal{T}|$, $|\mathcal{X}'|$, and $|\mathcal{X}|$ respectively, then $\dim _{t'}(\mathcal{T}'_{x'}) = \dim _{t}(\mathcal{T}_ x).$

Proof. Both sides can (by definition) be computed as the dimension of the same fibre product. $\square$

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