An affine scheme is a locally ringed space isomorphic to the spectrum Spec("R") of a commutative ring "R". A scheme is a locally ringed space "X" admitting a covering by open sets "U""i", such that each "U""i" (as a locally ringed space) is an affine scheme. In particular, "X" comes with a sheaf "O""X", which assigns to every open subset "U" a commutative ring "O""X"("U") called the ring of regular functions on "U". One can think of a scheme as being covered by "coordinate charts" which are affine schemes. The definition means exactly that schemes are obtained by gluing together affine schemes using the Zariski topology.