This chapter provides a diachronic corpus-based analysis of a vitally important, yet currently very under-researched, legal genre: the patent specification. The empirical focus of the analysis is on changes in the rhetorical move structure of the patent specification genre over the first 150 years of its existence, as represented by a corpus of British patent specification texts ranging from 1711 to 1860. The methodology of the study involves reducing each exemplar text in the corpus to a simple letter code string representing its rhetorical move structure, and then submitting the resulting list of code strings to two kinds of computational analysis: one focusing on the frequency and distribution of individual rhetorical moves over the period of the study, and the other focusing on the frequency and distribution of the different move sequence types attested in the corpus. It is argued that the findings of the study are consistent with, and may thus be interpreted as providing empirical support for, generalized evolutionary theoretical models of genre change.