Cousin (family relationship)

How kinship is measured

Cousin relationships are described by degree and removal. Degree indicates how many generations back the most recent common ancestor is counted: first cousins share grandparents, second cousins share great-grandparents, and so on. "Removed" expresses a generational offset: a first cousin once removed is one generation apart — for example, your first cousin’s child or your parent’s first cousin. These descriptions help map connections in family trees without implying direct descent from that common ancestor.

Common types and special cases

• First cousin: share grandparents.
• Second/third cousins: share great-grandparents or great-great-grandparents respectively.
• Once/twice removed: indicate generational difference between cousins.
• Half-cousins: share only one grandparent (their parents are half-siblings).
• Double cousins: occur when two siblings from one family marry two siblings from another family, so the children share both sets of grandparents.

History of the term

The English word "cousin" comes via Old French from Latin terms for kin on a parent's side. Historically, the term has sometimes been used loosely to indicate broader kinship or close family friends; modern genealogy uses precise formulas to identify the nearest common ancestor and generational distance.

Social, legal and biological importance

Cousin relationships matter in many areas: inheritance and succession rules can distinguish degrees of kinship; some legal systems place limits on marriage between close relatives, often treating first cousins differently from more distant cousins. From a biological perspective, closer cousins share a higher proportion of genes than unrelated people, which can influence the assessment of hereditary risks; for example, first cousins share more genetic material than second cousins, but precise percentages vary and are sometimes cited in genetic counseling.

Using cousin relationships

People use cousin terminology when building family trees, researching ancestry, or describing social bonds within extended families. A clear understanding of degree and removal helps convert a pedigree chart into a verbal description: tracing back to the nearest common ancestor and counting generations gives an unambiguous classification. Genealogists and legal authorities may use formal charts and rules to resolve ambiguous or culturally variant uses of the word ancestor and to distinguish collateral relatives from direct descendant lines.

For further reading about kinship terms and diagramming family relationships see resources on genealogy and kinship systems (family charts, aunt/uncle definitions, uncle relationships).