Raymond Harris-Northall uses the distinctive features of generative phonology to present synchronic and diachronic rules, but exploits the evidence of the history of the Spanish sound system to show that the ‘simplicity’ required by the generativists often obscures rather than illuminates the way in which changes develop from small beginnings. Instead, he illustrates the essential need to recognise the relevance of syllable structure constraints, which, allied with a complex but justified and helpful presentation of the relevant strength hierarchies, allow him not only to describe but to explain how changes began in the most suitable phonetic environments and then spread to subsequent items on the hierarchy over time. The result is the most serious and detailed application of strength hierarchy theory that has yet been made to a coherent body of historical data from two millennia of attestations, data that is interesting in its own right, and amenable to the theory.