Lemma 34.3.2. Let T be a scheme.
If T' \to T is an isomorphism then \{ T' \to T\} is a Zariski covering of T.
If \{ T_ i \to T\} _{i\in I} is a Zariski covering and for each i we have a Zariski covering \{ T_{ij} \to T_ i\} _{j\in J_ i}, then \{ T_{ij} \to T\} _{i \in I, j\in J_ i} is a Zariski covering.
If \{ T_ i \to T\} _{i\in I} is a Zariski covering and T' \to T is a morphism of schemes then \{ T' \times _ T T_ i \to T'\} _{i\in I} is a Zariski covering.
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