Although usage-based linguistics emphasises the need for studies of language change to take frequency effects into account, there is a lack of research that tries to systematically model frequency effects and their relation to diffusion processes in language change. This monograph offers a diachronic study of the change in Spanish perfect auxiliary selection between Old and Early Modern Spanish that led to the gradual replacement of the auxiliary ser be with the auxiliary haber have . It analyses this process in terms of the interaction between gradience, gradualness, and the...