The Cairo Genizah has preserved a vast number of medieval and post-medieval letters written in the Jewish variety of Arabic. The linguistic peculiarities of these letters provide an invaluable source for the understanding of the history of the Arabic language and the development of Arabic dialects. This work compares and contrasts various linguistic features of Judaeo-Arabic letters from different periods, and is one of the first studies to present a comprehensive linguistic investigation into non-literary Judaeo-Arabic. Its main focus is to provide an extensive diachronic linguistic description, while distinguishing between features of epistolary Arabic and vernacular phenomena. This study should be of interest to anyone working on the Arabic language, sociolinguistics, general historical linguistics and language typology.