The étale topology can be defined using slightly less data. First, notice that the étale topology is finer than the Zariski topology. Consequently, to define an étale cover of a scheme "X", it suffices to first cover "X" by open affine subschemes, that is, to take a Zariski cover, and then to define an étale cover of an affine scheme. An étale cover of an affine scheme "X" can be defined as a jointly surjective family {"u"α : "X"α → "X"} such that the set of all α is finite, each "X"α is affine, and each "u"α is étale. Then an étale cover of "X" is a family {"u"α : "X"α → "X"} which becomes an étale cover after base changing to any open affine subscheme of "X".