A longitudinal test of the Shevky-Bell model of increasing scale, using data from the U. S. for the last century shows the accuracy of the social rank and urbanization axes as aspects of increasing scale, but indicators selected from this model fail to confirm the segregation axis as a trend related to scale. The Shevky-Bell model of sub-area differentiation which its authors developed for census tract units was applied to the description of other sub-area units. The social rank axis appeared for each area unit, the urbanization axis was barely discernable, and the segregation axis appeared to be a general factor rather than an independent axis of differentiation. It is suggested that the indicators for the segregation axis are measuring the fundamental process of functional specialization of sub-areas, while the other two axes describe the factors around which the specialization of areas is organized. Furthermore it appears that the axes of sub-area differentiation shift as the sub-area unit of analysis is changed.