Definition
In Roman law, a recognized but legally inferior form of conjugal union, commonly translated as "natural marriage," as distinguished from the *justae nuptiae* or *justum matrimonium* — the formal civil marriage. Concubinatus was an informal, unsanctioned cohabitation that carried a degree of social and legal acknowledgment without conferring the full rights and status of civil matrimony.
The institution existed primarily to accommodate those who lacked the *jus connubii* — the legal capacity to contract a Roman civil marriage. This included certain classes of persons barred from formal marriage by reason of social status, citizenship, or legal disability. While concubinatus was tolerated and to some extent regulated by Roman law, it remained a second-tier relationship: the parties were not fully husband and wife in the civil sense, children born of the union were not subject to paternal power (*patria potestas*) in the same manner as those born of civil marriage, and the legal consequences flowing from civil matrimony — inheritance rights, legitimacy of offspring, and the wife's elevated status — did not automatically attach.
Common Language
Modern common usage (Wiktionary): A type of non-marital monogamous union in Ancient Rome.
Historical common usage (Webster's 1913): Not listed as a distinct entry; the related term *concubine* is defined as a woman who cohabits with a man without being his wife.
The common understanding of concubinage — a kept mistress or secondary partner existing alongside a lawful marriage — does not accurately describe concubinatus. In Roman law, concubinatus was typically a monogamous, exclusive union, not a supplementary relationship. Researchers reading historical sources that translate concubinatus loosely as "concubinage" should not import modern or medieval assumptions about secondary or polygamous arrangements.
Common Confusion
Concubinatus is sometimes conflated with the modern legal concept of common-law marriage or informal marriage. The resemblance is superficial. Common-law marriage in Anglo-American jurisdictions can, where recognized, produce the full legal consequences of formal marriage. Concubinatus in Roman law explicitly did not. The distinction was not merely ceremonial but substantive: legal rights of inheritance, legitimacy, and family structure turned on whether a union was *justum matrimonium* or concubinatus.
The term should also not be confused with *contubernium*, the Roman term for a cohabiting union between slaves, who had no legal capacity to marry at all. Concubinatus involved at least one free person with some legal standing; contubernium was beneath even that threshold.
Why It Matters in Research
This term appears almost exclusively in sources dealing with Roman law, civil law jurisdictions tracing their roots to Roman foundations, and ecclesiastical law history. Researchers will encounter concubinatus most often in:
— **Comparative and civil law treatises**: Scholars tracing the lineage of marriage law in Continental European and Louisiana-derived systems frequently discuss concubinatus as a historical predecessor or contrast point.
— **Canon law history**: The Catholic Church's effort to define and enforce formal marriage standards during the medieval period was partly a reaction against informal unions tolerated under Roman and Germanic custom. Sources discussing the Council of Trent's marriage reforms may invoke concubinatus as background.
— **Legitimacy and inheritance disputes in historical records**: In jurisdictions with civil law influence, older legal proceedings touching on the status of children born outside formal marriage may use terminology derived from the Roman distinction between civil marriage and concubinatus.
**Trap for researchers**: The word "natural" in the historical definitions ("natural marriage") is a term of art drawn from Roman law categories, not a compliment or an indication of social approval. It denoted the absence of civil formality, not the presence of natural-law legitimacy. Do not read "natural marriage" in this context as equivalent to any modern usage of that phrase.
**Jurisdictional trap**: In Louisiana legal history and in historical Spanish and French colonial law sources, informal unions occupied a contested space influenced partly by Roman-derived concepts. A researcher encountering concubinatus in those sources should look carefully at the specific legal context — rights being claimed, the period, and the applicable law — before drawing analogies to Roman law proper.
Historical Dictionary Support
The four source dictionaries are in close agreement on the core definition: concubinatus is the Roman "natural marriage" as opposed to the *justum matrimonium*. Rapalje & Lawrence follows Bouvier directly and adds nothing independent.
Bouvier provides the most substantive treatment of the two, noting that concubinatus was the only form of conjugal union available to those lacking the *jus connubii*, and explicitly flagging that it produced none of the important legal consequences of civil marriage — paternal power being the example given (the entry appears to continue beyond the excerpted text). Black's in both its first and second editions offers the compact formulation — "informal, unsanctioned, or 'natural' marriage" — without elaborating on legal consequences.
What the historical dictionaries collectively understate is the regulatory dimension: Bouvier gestures at it ("authorized and regulated by law") but does not develop it. Roman law under Augustus and later emperors did impose some structure on concubinatus — restricting who could enter such unions, imposing obligations toward children, and distinguishing it from mere promiscuous cohabitation (*stuprum*). Researchers working in Roman law scholarship proper should consult primary Romanist sources rather than relying on these dictionary treatments as complete accounts.
Jurisdictional Note
Concubinatus is a term of Roman law without direct equivalent in modern common-law systems. It retains relevance in civil law scholarship, ecclesiastical law history, and the historical law of jurisdictions with Roman or civil law foundations. It carries no operative legal meaning in current American, English, or Commonwealth law.