The 'Oxford Grid' is a term used to denote a method of displaying homologous loci from two species. It may also be used as a basis for historical inferences relating to co-ancestry. These uses are discussed with special reference to man and mouse. It is inferred that as few as 30 reciprocal translocations are sufficient to explain the differences defined by the present grid and that the telocentric karyotype, through which the mouse differs from both man and many closely related rodents, including the rat, must have evolved mainly through the formation and suppression of centromeres rather than through pericentric inversions. Mouse and man share the widest distribution of any mammal and their success appears to be related to being omnivorous with behavioural modifications allowing a wide range of habitat.