Definition
Affines (singular: affine) are persons connected to one another by marriage rather than by blood. In civil law usage, the term encompasses two distinct meanings:
1. Persons related by marriage — either the spouses themselves or the relatives of each spouse who become connected to the other spouse's family through that marriage. A husband's siblings, for example, are affines to the wife.
2. Neighbors — specifically, persons who own or occupy adjoining parcels of land. This second meaning, drawn from Roman law (Digest 10.1.12), rarely appears in modern legal usage and is confined almost entirely to historical civil law sources.
The governing maxim — affinis mei affinis non est mihi affinis — establishes that affinity does not extend indefinitely along a chain of marriages. The person who is related by marriage to someone already related to you by marriage is not your affine. Affinity, in other words, does not compound: it is a one-step relationship.
Common Language
Wiktionary: Plural of affine.
The common English word "affine" carries geometric and mathematical senses (as in affine transformation) that are entirely unrelated to the legal term. A researcher encountering "affines" in a legal or historical document should not import any mathematical meaning. The legal term is purely relational — it describes people, not spatial or algebraic properties.
Common Confusion
Affines is frequently encountered alongside, and sometimes conflated with, consanguines — persons related by blood. The distinction matters enormously in family law because legal prohibitions on marriage, inheritance rules, and certain evidentiary privileges historically turned on whether a relationship was one of affinity (marriage-based) or consanguinity (blood-based). Affinity dissolves on death or divorce in some legal systems; consanguinity does not. Researchers should not treat the terms as interchangeable.
Why It Matters in Research
The primary research significance of affines arises in three areas:
Marriage prohibitions. Canonical and civil law both maintained tables of affinity listing which affines could not marry one another. These tables varied by jurisdiction and era. Researching whether a historical marriage was valid — or voidable — often requires consulting the applicable affinity table, not merely a modern definition.
Inheritance and succession. Some intestacy regimes and testamentary frameworks gave affines limited or conditional rights. Others excluded them entirely in favor of consanguines. A document that uses "affines" to describe a class of potential heirs must be read against the specific jurisdictional rules in force at the time.
The neighbor meaning. The civil law sense of affines as adjoining landowners appears in Roman law sources and in early common law texts that drew on the Digest. Researchers working with land partition actions, boundary disputes, or early property records may encounter this usage. Black's cites Digest 10.1.12 directly. Treating this as a family law term when the context is property would be a significant misreading.
The maxim affinis mei affinis is also substantively important: courts and canonists relied on it to limit the reach of affinity prohibitions. Where a party claimed that a proposed marriage was barred by affinity, the maxim cut off the argument at one remove. Researchers tracing marriage litigation or dispensation records should understand how courts applied this limiting principle.
Historical Dictionary Support
Both the first and second editions of Black's Law Dictionary reproduce identical definitions with only minor typographical variation — a reliable sign that the term was settled in meaning by the time Black's first appeared and did not evolve between editions. Both editions cite Calvin (Calvinus's legal lexicon) for the marriage-connection meaning and the Digest for the neighbor meaning, and both reproduce the affinis mei affinis maxim with attribution to Shelford on Marriage and Divorce.
What the historical dictionaries do not address: the procedural and evidentiary consequences of affinity relationships (witness competency, testimonial privilege), the dissolution of affinity upon death of the connecting spouse, or the significant variation between canon law and civil law in how far affinity extended. Researchers relying solely on Black's for an understanding of affinity's legal reach will find the entry a starting point only.
Jurisdictional Note
Canon law and civil law traditions treated the reach of affinity differently, and American states have not been uniform. Some states historically extended affinity prohibitions to collateral relatives of a spouse; others confined them to lineal relationships. Modern American law has substantially narrowed or abolished affinity-based marriage prohibitions, but historical research — particularly involving ecclesiastical records or pre-twentieth-century domestic relations cases — requires attention to the rules applicable in the specific jurisdiction and period.
Encyclopedia Cross-Reference
Marriage — Capacity to Marry (Age, Consanguinity, Affinity), The Law Mind Family Law Encyclopedia