Notionally, lineages are exclusive in their membership. In practice, however, many cultures have methods for bestowing lineage membership on individuals who are not genetically related to the lineage progenitor. The most common of these is adoption, although other forms of fictive kinship are also used. Lineages are normally corporate, meaning that their members exercise rights in common and are subject to obligations collectively.

Lineage structure may be regarded as a branching process, as when two or three founders of small lineages are represented as brothers or sisters. The groups thus constitute a single larger lineage in which the smaller groups are segments. This structure may lend stability to a society; the lineages are considered permanent groups and thus perpetuate concomitant political and religious relationships over time. In societies lacking central political authority, territorial groups often organize themselves around lineages; as these are usually exogamous (exogamy), or out-marrying, marriage becomes a means of bringing together otherwise unrelated groups.