Given the space formula_9 with the Zariski topology, the structure sheaf "O""X" is defined on the distinguished open subsets "D""f" by setting Γ("D""f", "O""X") = "R""f", the localization of "R" by the powers of "f". It can be shown that this defines a B-sheaf and therefore that it defines a sheaf. In more detail, the distinguished open subsets are a basis of the Zariski topology, so for an arbitrary open set "U", written as the union of {"D""fi"}"i"∈"I", we set Γ("U","O""X") = lim"i"∈"I" "R""fi". One may check that this presheaf is a sheaf, so formula_1 is a ringed space. Any ringed space isomorphic to one of this form is called an affine scheme. General schemes are obtained by gluing affine schemes together.