This article examines the meaning of the Tibetan kinship term źao (“maternal uncle”, “father-in-law” or “wife-giver”) as it applied to the maternal relatives of the Tibetan royal line during the period of the Tibetan empire (c.600—c.850). Based on a close examination of several Old Tibetan sources, the article demonstrates that the appellative źao was lent to members of an aristocratic clan when one of its ladies gave birth to a Tibetan emperor (or upon his subsequent accession to the throne), and that the title was retained for at least four generations thereafter. The investigation also reveals a proscription governing the marriage practices of the Tibetan royal line: no heir-producing marriage with a single maternal clan was permitted until a certain number of generations had passed since an earlier such union. The ramifications of this practice are then considered alongside a modern parallel, and the paper closes with a few extrapolations concerning the social structure of the Tibetan empire