Curves have been constructed showing the variation of mean density with mass of planetary models of terrestrial type. The models are taken to have the same composition at zero pressure as the Earth, and are based on the pressure–density relations of an Earth Model B and on the hypothesis that the outer part of the central core of this model is a pressure modification of the mantle. Two curves are shown, corresponding to the assumptions that the Earth–s inner core is caused by chemical and pressure changes, respectively. The curves are compared with Ramsey's original curve which was based on an Earth Model A and assumed the density distribution to be determined essentially by pressure alone.