Although tractors are now used in nearly every agricultural field operation and in the production of nearly all crops, they first developed with much more limited application. Early diffusion was accordingly rapid in these narrower applications, but limited in scope until tractor technology generalized. The sequence of diffusion is consistent with a model of R&D in specific- versus general-purpose attributes and with other historical examples, suggesting that the key to understanding technology diffusion lies not only in explaining the number of different users, but also in explaining the number of different uses.