Lineal consanguinity describes the direct blood relationship between people who descend one from another in a straight line, such as between parent and child, grandparent and grandchild, and so on upwards or downwards. It excludes collateral relatives (for example siblings, aunts, uncles and cousins), who share a common ancestor but do not descend directly from one another. In UK and Irish legal practice, lineal consanguinity is particularly relevant to the law on marriage and civil partnership prohibitions, succession and wills, conflicts of interest, and certain family law and adoption issues. It is commonly expressed in terms of “degrees” of relationship (first degree: parent–child; second degree: grandparent–grandchild, etc.). The concept is recognised across England and Wales, Scotland, Northern Ireland and Ireland, with broadly consistent usage, although specific statutory wording and treatment of degrees of relationship may differ between jurisdictions and legislative schemes. Some statutes refer expressly to “lineal ancestor” or “lineal descendant” rather than “lineal consanguinity”, but the underlying principle is the same: a direct blood line connecting two individuals.