Let us suppose that city size class 1 is defined so that it contains only the primate city (i.e. f1 can denote the size of the largest city as in the rank-size rule).Then clearly (4.8) is of the same form as the rank-size rule given in equation (4.1), and may be compared with equation (2.17) in form. Thus a city-size distribution following the rank-size rule may be regarded as the most probable distribution, the state in which entropy has been maximized—'a