A descent group is any social group in which membership depends on common descent from a real or mythical ancestor. Thus a lineage is a unilineal descent group in which membership may rest either on matrilineal descent (patrilineage) or on matrilineal descent (matrilineage). In a cognatic descent, all descendants of an ancestor\ancestress enjoy membership of a common descent group by virtue of any combination of male or female linkages.
However, cognatic descent is sometimes used synonymously with either 'bilateral' or 'consanguine descent. A clan is a unilineal descent groups the members of which may claim either partilineal (Patriclan) or matrilineal descent (Matriclan) from a founder, but do not know the genealogical ties with the ancestor\ancestress.
A phratry is a grouping of clans which are related by traditions of common descent. Mythical ancestors are thus common in clans and phratries. Totemic clans, in which membership is periodically reinforced by common rituals such as sacred meals, have been of special interest to social anthropologists and sociologists of religion. Where the descent groups of a society are organized into two main divisions, these are known as moieties (halves). The analysis of descent groups is crucial for any anthropological study of pre-industrial society, but in most Western industrial societies the principle of descent is not prominent and descent groups are uncommon.