Definition
Collateral consanguinity is the relationship by blood between persons who share a common ancestor but are not descended from one another. Two brothers share the same father but neither descends from the other; two cousins share the same grandparents but stand on parallel branches of the family tree. This is the defining characteristic: the kinship runs sideways through the family, not up and down through a direct line of descent.
This stands in contrast to lineal consanguinity, which describes the straight vertical relationship between ancestors and descendants — parent to child, grandparent to grandchild. Collateral relatives are, by definition, those who can only reach their common ancestor by first climbing up one branch of the family tree and then descending another.
The degree of collateral consanguinity is calculated by counting the steps (or generations) between the two relatives through the common ancestor. Full siblings are collateral relatives of the second degree (one step up to the shared parent, one step down). First cousins are of the fourth degree (two steps up to the shared grandparent, two steps down). Different legal systems have applied different counting methods — the civil law method and the canon law method produce different numerical results for the same relatives — and the applicable method must be identified before any degree calculation means anything in a legal context.
Why It Matters in Research
The concept sits at the intersection of inheritance law, marriage prohibition, and intestacy, making it one of the most practically consequential kinship classifications in historical legal sources.
In inheritance and intestacy research, collateral consanguinity determines who takes when a decedent leaves no lineal descendants and no spouse. Early English common law and American statutes built elaborate hierarchies of collateral kin — siblings first, then half-siblings, then uncles and aunts, then cousins — each described by degree of consanguinity. When researching estate disputes in historical case law or early American statutes, you will encounter collateral consanguinity used as a threshold concept: kin below a certain degree were simply excluded from inheritance.
The most active modern application is in marriage prohibition law. Every jurisdiction prohibits marriage between persons within certain degrees of consanguinity. Because the rules apply differently to lineal versus collateral relatives (most jurisdictions prohibit first-cousin marriages or draw the line at some collateral degree), the lineal/collateral distinction is not merely taxonomic — it is outcome-determinative in annulment proceedings, legitimacy disputes, and void/voidable marriage analysis.
Watch for degree-counting inconsistencies across historical sources. Pre-nineteenth-century English common law sources, ecclesiastical sources, and American state statutes frequently used different counting conventions without flagging the difference. A "third degree" collateral relative in a canon law source is not the same relationship as a "third degree" collateral relative in a common law source. This creates genuine traps in historical research: two sources may appear to contradict each other on whether a particular relationship was prohibited or excluded, when they are simply using different arithmetic.
Historical Dictionary Support
Black's Law Dictionary follows Blackstone closely, drawing the perpendicular line / lateral line contrast as the organizing metaphor: lineal consanguinity is the vertical line (linea recta), collateral consanguinity is properly the horizontal or lateral relationship. Black's treatment is clear and structural, anchored in 2 Bl. Comm. 203.
Burrill's Law Dictionary reaches the same definition through a slightly different framing, emphasizing the shared stock or ancestor — "whether near or remote" — as the unifying feature, and illustrating the concept with the two-brothers and two-cousins examples. Burrill also supplies the Latin phrase cognatio à latere, literally "kinship from the side," which captures the lateral structure of the relationship. Both dictionaries agree completely on substance; the difference is one of emphasis.
Neither source addresses the degree-counting divergence between civil law and canon law methods, nor the resulting interpretive complications in American statutory research. Researchers relying solely on these entries for historical work should supplement with the governing statutory framework applicable to the jurisdiction and period at issue.
Jurisdictional Note
American states vary significantly on which degrees of collateral consanguinity void a marriage. Most states prohibit marriage between first cousins (fourth degree civil law); some permit it; a smaller number prohibit second cousins as well. Historical statutes are even more variable. Research into prohibited degrees must be grounded in the specific state's law at the specific time in question.
Encyclopedia Cross-Reference
The Law Mind Family Law Encyclopedia: Marriage — Capacity to Marry (Age, Consanguinity, Affinity) [family_2]