An X-ray standing wave (XSW) field is created by interference between an X-ray beam impinging on a sample and a reflected beam. The reflection may be generated at the Bragg condition for a crystal lattice or an engineered multilayer superlattice ; in these cases, the period of the XSW equals the periodicity of the reflecting planes. X-ray reflectivity from a mirror surface at small incidence angles may also be used to generate long-period XSWs.