Bilateral cross cousin marriage begins with an initial situation of exchange marriage. Two men marry each other's sisters to establish a basis for a long term alliance.
Marriages in Initial Generation
Note that marriage is indicated by lines joining couples from below and not by equal signs.
The cross cousin marriage rule is applied in the next generation. Ego is expected to marry his bilateral cross cousin, who is at the same time both his mother's brother's daughter and father's sister's daughter, because of the intermarriage between sets of parents.
Marriages in Second Generation
An reapplication of the cousin rule in the third generation, continues the pattern of exchanges in the previous generation.
Marriages in Third Generation
The regular application of the bilateral cross cousin marriage rule creates a permanent alliance between a pair of lineages (A and B) through the continuous intermarriage between the men of A and the women of B and vice versa. This arrangement is often further articulated into dual organizations or moiety systems, in which basic social units are composed of paired groups linked by marriage relationships. The Yanomamo of Amazonia provide an example. Their basic social unit is the village, composed of between 50 and 200 inhabitants. Each such settlement is composed of two localized patrilineages or, in effect, patrilineal moieties. The lineages are closely bound into a unified social order by intermarriage through the firm imposition of the bilateral cross cousin rule. (See Yanomamo marriage).
Further elaborations on the bilateral cousin principle have been developed, including the section systems of Australia. In these situations variant cousin marriage rules create sections of 4 and sometime 8 units linked by regular patterns of intermarriage.