Definition 85.27.1. Let a : Y \to X be a morphism of simplicial schemes. We say a is cartesian, or that Y is cartesian over X, if for every morphism \varphi : [n] \to [m] of \Delta the corresponding diagram
\xymatrix{ Y_ m \ar[r]_ a \ar[d]_{Y(\varphi )} & X_ m \ar[d]^{X(\varphi )}\\ Y_ n \ar[r]^{a} & X_ n }
is a fibre square in the category of schemes.
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