Definition
Affinitas affinitatis is a Latin term from the civil and canon law traditions meaning, literally, "affinity of affinity" — the relationship that arises between the relatives of two persons joined by marriage, where those relatives share neither blood (consanguinity) nor direct affinity with each other. It describes the connection that exists, for example, between a husband's brother and the wife's sister: they are connected through the marriage but are neither kin by blood nor related to each other by direct affinity.
The distinction matters because classical affinity (affinitas) described the relationship between a spouse and the blood relatives of the other spouse — the husband and his wife's brother, for instance. Affinitas affinitatis extended that relational web one step further, capturing the lateral connections between each spouse's family members who have no independent blood or affinity link to one another.
Common Confusion
Affinitas affinitatis is frequently collapsed into its parent concept, affinity (affinitas), by researchers working with older legal materials. The two are distinct. Affinity is the direct tie between a spouse and the other spouse's kin. Affinitas affinitatis is the indirect, collateral connection between the two families' respective members — a second-order relationship with no equivalent in modern domestic relations terminology. Many historical sources use "affinity" loosely enough to encompass both, which creates interpretive problems when the legal consequence (prohibited degrees of marriage, inheritance eligibility, competency as a witness) depends on which relationship is actually meant.
Why It Matters in Research
This term appears almost exclusively in canon law discussions of prohibited degrees of marriage and in civil law treatises addressing competency, inheritance, and testimonial disqualification. It has no meaningful survival in modern Anglo-American statutory law, where marriage prohibitions are defined by specific relationships rather than by civil law categories.
Researchers working in ecclesiastical court records, pre-Reformation marriage law, or early American and colonial legal materials will encounter this term in the context of impediments to marriage. Canon law treated affinitas affinitatis as a potentially disqualifying relationship within certain degrees, though the church's treatment of this category varied across periods and was substantially narrowed over time. By the time Bouvier published his dictionary in the nineteenth century, the term was already largely of historical interest, retained for its doctrinal precision rather than active legal application.
The principal trap in historical sources is conflation. Writers in the early common law period, and even some civil law commentators, used affinitas loosely to cover both direct affinity and this second-order relationship. When reading a source that turns on the question of whether a particular relationship was close enough to bar marriage or create disqualification, a researcher must determine which category of relationship was actually intended. Erskine's Institutes, cited by Bouvier, provides one of the cleaner treatments for Scots law, which maintained civil law categories longer than English common law did.
There is also a jurisdictional dimension worth noting: American courts in the nineteenth century occasionally engaged with this concept in cases involving testimonial competency (whether a person was disqualified as a witness due to relationship to a party) and in statutory construction of marriage prohibition statutes that used civil law vocabulary. These applications are sparse but real, and the term may surface in reported opinions from that period.
Historical Dictionary Support
Bouvier and Rapalje & Lawrence agree on the core definition: a connection that is neither consanguinity nor affinity, illustrated by the husband's brother and the wife's sister as the paradigm case. Both trace the concept to the civil law maxim preserved in Rapalje: affinitas dicitur, cum duae cognationes, inter se divisae, per nuptias copulantur — affinity is said to arise when two families, otherwise separate, are joined through marriage and one approaches the bounds of the other. The definition in both dictionaries is skeletal, reflecting the term's receding practical importance by the time these works were compiled. Neither source engages with the canon law history of the concept's use in prohibited degrees analysis, which is where the term did its most consequential legal work. Researchers needing fuller treatment must move beyond these dictionaries to civil and canon law sources directly.
Jurisdictional Note
This term belongs to the civil and canon law traditions and never became a term of art in the common law. Its relevance in American and English legal research is historical and interpretive rather than operational — it appears in sources and commentary, not in modern statutes or court rules.