# Is there any identification assumption for IV?

I am asking this because I know there are IA's for cross sectional estimators and D-in-D estimators, but am unsure if there are any for IV estimation. Does anyone know if there is or isn't, and if there is - what would it be?

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The instrumental variable $Z$ must be:
2. Correlated with the regressor $X$ for which it is to serve as instrument (IV relevance).