This paper describes three contexts of differentiation (arabness-‘ajamness / Turks-Kurds / Kurdish territory) and their evolution during the Middle Ages from a rudimentary definition of “Kurd” to a more diversified conception. It interprets them through three lenses: micro-social (actor subjectivity), meso-social (competition/transaction between groups or categories), and macro-social (state ascription). In medieval Arabic sources, the use of “Kurd” (kurd, pl. akrâd) generally does not fit into contemporary social and ethnic categories. Many scholars have resolved the issue by simplistic statements: “The term Kurd is not an ethnic term, it is a social one”, or: “Kurd is synonymous with Iranian nomads”. The process of categorization is multiform, originating from several sources and resulting in a polysemic category. It is undergirded by social, political and intellectual environments that implement many constructions. As Catherine Quiminal writes: “While categorizing one doesn’t only arrange objects in pre-given classes of belonging. One selects a principle of classification which defines the situation.” Like other ethnonyms (‘arab, turk, furs, and ‘ajam) “Kurd” is used in various contexts of meaning defined by evolving conceptions of difference. From al-J??i? to Ibn Khald?n, Arabic medieval authors – whether they vaguely mention “Kurds” or insert them in a sophisticated social theory – articulate conceptions of “Kurdish” difference. Three main (rhizomatic) models appear: First, during the classical era (700s-1000s AD) a binary framework of reference developed, situating Kurdishness in--between “`arabness” and “`ajamness”. Second, during the following centuries (1100s-1300s AD) the collapse of an exclusively Arab empire enabled a more diverse conception of difference, which included the Kurds in the Khald?nian register of ‘bedouinity’ or the broader narrative of “barbarians” of the edges. Parallel to these processes of utterance a rather paradoxical one crystallized the category “Kurd”. Essentially recognized for their military skills, the Kurds seemed to enter the city (civitas) along with the Turks. While associated with Turks within military oligarchies, “Kurds” reinforced a process of differentiation necessary to compete for resources. Third, my paper considers the geo-ethnic dimension of Kurdish identity. In Arab geographers’ and chroniclers’ spatial conception of the medieval East, some designations referred clearly to a Kurdish presence (Bilad al-Akrâd, Zûzân al-Akrâd, Kurdestân, etc.) in Upper Mesopotamia. This dimension emerged from the frontier culture among the Kurds that has lasted until today, an in-between situation that actors have shaped and exploited, and that is shaping Kurdishness in return, due to the intimate association of people and environment.