The study of universes which are homogeneous but not isotropic. Standard cosmological models assume that the Universe, on cosmological scales, is both homogeneous (looks the same at every place) and isotropic (looks the same in every direction)—that is, it conforms to the cosmological principle. If the assumption is relaxed, so that the former condition holds but not the latter, then the allowed solutions of the equations of general relativity are called Bianchi models, after the Italian mathematician Luigi Bianchi (1856–1928).