Chapter 4 uses the thirteenth-century Ibadi prosopography called the Kitab al-tabaqat by Abu al-ʿAbbas al-Darjini to show that this period witnessed the formalization of the Ibadi prosopographical tradition in several ways. These included the written institutionalization of a council-rule system, the structural arrangement of Ibadi scholars from the past into generations of fifty years (tabaqat), the linguistic triumph of Arabic in written scholarship, and a further move toward manuscripts as tools for the transmission of knowledge alongside their oral equivalents. These steps toward formalization likewise mirror the changes in the political and religious landscapes of the Ibadi archipelago out of which the book emerged. Finally, the chapter shows how the written network of Kitab al-tabaqat represents a contracted and refined version of its predecessors.