The eighteenth century was a key period in the establishment of standard modern English. This period, referred to as the Late Modern English period, witnessed the publication of an unprecedented number of normative works aiming to define ‘correct’ English. Joseph Priestley (1733–1804) is best known as a scientist and theologian, but his Rudiments of English Grammar, first published in 1761 is an important work in the wave of English normative grammars of the late eighteenth century. Using a multi-disciplinary approach, this book investigates Priestley’s role as a codifier of the English language. The author demonstrates that the influence of Priestley’s grammar on the language has been underestimated and merits re-evaluation. Priestley’s ideas on grammar are related to his broader philosophical thinking. It is shown that, although Priestley is usually seen as one of the few descriptive grammarians of the period, his grammar also contains decidedly prescriptive elements, and that his adherence to the force of usage should be qualified. In addition, Priestley’s usage is compared to the rules in his grammar using a corpus of Priestley’s personal correspondence, created for this study. This book is of interest to sociohistorical linguists studying Late Modern English and historical linguists in general, as well as to social historians and anyone interested in Joseph Priestley or the Late Modern period in England.