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Why might a kinship system use the same term for a father and his brother?

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Large illustrated genealogical tree with detailed branches and leaves, featuring family names and a central family crest, representing the Lippincott family lineage. — Genealogy Genealogical tree of the Richard and Abigail Lippincott family in America, constructed and published by Charles Lippincott, 1880.

In anthropology, the practice of using the same term for a father and his brother is a hallmark of a classificatory system of kinship terminology. In these systems, collateral relatives (those not in a direct line of descent) are grouped with lineal relatives (those in a direct line of descent) who share the same sex and generation.1

This specific pattern is often associated with bifurcate merging terminology. In this arrangement, a father and his brother are identified by the same term because they are both male relatives of the ascending generation within the same lineage. Conversely, a mother’s brother is often distinguished with a different term, such as uncle, because he belongs to a different descent group.21

It is important to note that using a single term does not imply that members of a society are confused about biological relationships. Rather, these terms reflect social categories and the structure of descent groups, which organize rights, responsibilities, and marriage patterns within a community.1