Khanty kinship is based on a patrilineal descent system, with aspects of cognatic (nonunilineal) descent pragmatically recognized and reflected in kin terms. Each person traditionally had a tribe, through kinship and regional affiliation, although sources vary on its definition. Tribes were centered around the Kazym, Vasiugan, Vakh, Irtysh, and northern Ob river systems. Ob peoples were Asiakh, from which "Ostyak" may have derived. Exogamous patrilineages ( puch or poch ) had totemic identities and traced descent to a founding male ancestor. Larger social categories, syr, also provided a basis for identifying marriage partners. These exogamous phratries, sometimes termed moieties, crosscut Khanty, Mansi, and even Selkup divisions. Two main groups, Por and Mos, dominated the Ob River Basin. Most Khanty still know these identities, and some follow marriage rules associated with them.