Using historical and numerical analysis and the five-part schema, this study finds that over the past 50 years structural modifications and adaptations by American cities have generally followed the standard S curve of the diffusion of innovation. In tests of Kaufman's and Hirshman's theories of epochs of change from representativeness to administrative efficiency, this study determines that mayor-council cities have, in a standard innovation diffusion S curve, adopted many of the key features of council-manager cities, increasing their administrative efficiency. At the same time, council-manager cities, again in an S curve, have adopted many of the key features of mayor-council cities, increasing their political responsiveness. Fewer cities are now either distinctly mayor-council or council-manager in form, and most cities are structurally less distinct, constituting a newly merged or hybrid model of local government—the type III city.