# Kerodon

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Remark 3.1.1.6. The collection of Kan fibrations is closed under pullback. That is, given a pullback diagram of simplicial sets

$\xymatrix@R =50pt@C=50pt{ X'_{} \ar [d]^{f'} \ar [r] & X_{} \ar [d]^{f} \\ S'_{} \ar [r] & S_{} }$

where $f$ is a Kan fibration, $f'$ is also a Kan fibration.