The diverse origins of the Nanai are reflected in their clan composition. In the nineteenth century, there were nineteen clans ranging in size from 40 to 60 persons up to 900. Since 1897 this Nanai clan structure has remained stable. All clans on the Amur are classified into two larger groupings—"downriver" (below Lake Bolon), consisting of the Samar, Gaer, Tumasli, most of the Kile, and Khodzher, and "upriver," consisting of the Perminkan, Donkan, most of the Bel'da, and others.
Each clan consisted of several branches of different origins, and in the larger clans, such as the Bel'da and Khodzher, the subclans numbered several dozen. Members of each clan were dispersed across a number of settlements (even members of the smallest clans lived in two or three villages), often at great distances from one another. Marriage was clan exogamous, making marriage alliances an important feature of Nanai cultural cohesion and cooperation. Clans with few members, those whose populations were reduced by epidemics, and migrant groups all sought unions with other clans that would benefit them. The fusion of several Nanai clans can be traced through both documentary information and legends.
Every Nanai clan was, in its origin, a complex union, the result of resettlement, fusion, enlargement, fission, and ties to other ethnic groups. Thus the development of each clan to the modern form was an extraordinarily complex process. The clans of the nineteenth century were also affected by more recent events, including the decline of some clans, the fragmentation of others, and the fusion of others. The largest clans were the Bel'da, Khodzher, Samar, and Kile. The fragments of disappearing clans were often eager to fuse with the Bel'da clan.
Small clans such as the Odzial, Saigor, Gaer, and others formed a distinctive type of alliance, the dokha, for mutual assistance. These clans were also exogamous, although a person in great need of a spouse for support, such as a widow, would sometimes be allowed to marry a man from another clan within the same dokha.
Alliances of various types were a vital component of Nanai culture and provided mutual assistance on a regular basis as well as an accompanying sense of security for individual Nanai. Mutual assistance was obligatory for all Nanai as well as outsiders; the Nanai, like their neighbors, often aided others from different ethnic groups.