The structure of a 30 nm thick pentacene film grown by molecular beam deposition on an oxygen passivated Cu(110) single crystal has been investigated ex-situ by X-ray diffraction methods. It is shown that pentacene crystallizes in two known bulk polymorphs with four unique crystal orientations. In all four cases, a principal pentacene direction is aligned along the surface corrugation provided by the Cu(110)-(2 x 1)0 reconstruction. Since overlayer and substrate form incommensurate lattices, the results cannot be understood by classical criteria of organic epitaxy and the importance of one dimensional alignment is discussed. (c) 2008 Elsevier B.V. All rights reserved.