Definition
*Reus promittendi* is a Latin term from Roman and civil law meaning "the party who is bound by a promise" or, more precisely, the debtor in an obligation — the one who has undertaken to perform. In the context of contract and obligation law, the *reus promittendi* is the promisor: the party who has made the binding undertaking and against whom performance may be demanded.
The term operates in contrast to *reus stipulandi*, which designates the creditor in an obligation — the party to whom the promise runs and who may demand performance. Together, the two phrases identify the poles of an obligatory relationship under Roman law: one who is bound (*promittendi*) and one who may compel (*stipulandi*).
In the conflict of laws literature, the term appears in discussions of which law governs an obligation — specifically, the domicile or act of the promisor as a connecting factor for choice-of-law analysis.
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Common Confusion
*Reus promittendi* is occasionally confused with *reus* in its criminal law sense, where the word simply means "the accused" or "defendant." The two usages are unrelated. In criminal law, *reus* appears in the compound *actus reus* (the guilty act) and carries connotations of culpability and wrongdoing. In the obligation context, *reus promittendi* carries no such connotation — it denotes a party to a civil undertaking, not a criminal defendant. Researchers encountering *reus* in a historical source must determine from context whether the civil-obligation or criminal sense is intended.
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Why It Matters in Research
This term surfaces almost exclusively in civil law and conflict of laws materials. Researchers should be alert to several navigational points:
**Conflict of laws.** The primary modern research context for *reus promittendi* is private international law, particularly the older literature on the proper law of a contract or obligation. Story's *Commentaries on the Conflict of Laws* and Wharton's conflict of laws treatise both use the term when discussing which jurisdiction's law governs the promisor's obligation — a question framed around the acts and domicile of the *reus promittendi*. Researchers working in pre-twentieth-century conflict of laws materials will encounter it there.
**Civil law and Roman law sources.** The term belongs to the technical vocabulary of Roman obligatio doctrine, which organized contractual liability around the relationship between *reus stipulandi* (creditor) and *reus promittendi* (debtor). It appears in Justinian's *Institutes* and *Digest* and flows from there into the civilian tradition. Researchers in comparative law, Louisiana law, admiralty, or any jurisdiction with a strong civil law inheritance are more likely to encounter it than those working solely in common law sources.
**Absence from common law dictionaries.** Most common law legal dictionaries — including Bouvier and Black's — give the term minimal treatment or omit it. Its survival in English-language legal literature is largely through conflict of laws treatises and civil law commentary. A researcher who finds the phrase in a nineteenth-century American court opinion is almost certainly dealing with a court drawing on Story, Wharton, or a civil law source directly.
**Pair always with *reus stipulandi*.** The term has limited standalone meaning. Understanding the promisor-creditor dyad is necessary to make sense of how historical sources use either phrase.
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Historical Dictionary Support
Rapalje & Lawrence's entry is minimal, directing the reader to the general entry for *reus* and citing Story's *Conflict of Laws* (§ 401) and Wharton's conflict of laws work. This terseness is itself informative: the compilers treated the term as sufficiently self-explanatory within the civilian tradition that extended treatment was unnecessary, and they understood its primary legal utility to be in the conflict of laws context rather than domestic contract doctrine.
The citation to Story (§ 401) places the term in the discussion of where an obligation is formed and which law governs it — a passage concerned with the connecting factor of the promisor's act. The citation to Wharton reinforces this conflict of laws framing.
What the historical dictionaries do not supply is the full Roman law backdrop that gives the term its meaning. For that, researchers must turn to the civilian sources directly — the *Institutes* of Justinian, Pothier's *Treatise on Obligations*, or secondary works on Roman contract law. The English-language legal dictionaries of the nineteenth century largely assumed the reader already possessed that background.
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Encyclopedia Cross-Reference
The Law Mind Criminal Law Encyclopedia — *Elements of a Crime (Actus Reus and Mens Rea)* — addresses the criminal law sense of *reus* but is not directly relevant to *reus promittendi*. No Law Mind Encyclopedia entry directly covers Roman obligation doctrine or the conflict of laws framework in which this term operates. Researchers may find contextual background in the criminal law entries when tracing the broader history of the word *reus*, but should not expect those entries to illuminate the civil obligation meaning.
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