Definition
In Roman civil law, an adpromissor is a surety or guarantor who supplements the promise of the principal debtor by adding a parallel promise of his own. The adpromissor does not merely pledge collateral or assets — he makes an independent verbal commitment to perform the same obligation as the principal, standing alongside the debtor rather than simply behind him. The term is most closely associated with the formal system of suretyship developed under Roman law and received into Scots law, where it functioned as a species of fidejussion.
Common Language
Modern common usage (Wiktionary): A surety.
The gap between the colloquial "surety" and the civil law adpromissor is not merely semantic — it is structural. A modern surety may guarantee payment through a range of mechanisms, including pledges, bonds, and indemnity instruments. The adpromissor guaranteed by the specific act of adding a spoken promise (the adpromissio) to the principal's own verbal undertaking, a form tied to the Roman stipulatio. The common word "surety" erases this procedural specificity entirely, which matters when reading historical sources that use the terms interchangeably.
Common Confusion
ADPROMISSOR vs. FIDEJUSSOR: These terms are often used synonymously in historical legal sources, and Black's itself calls the adpromissor "a peculiar species of fidejussor." The distinction lies in technical Roman law classifications of accessory obligors. The adpromissor was the broader genus; the fidejussor was a later and more refined form that eventually displaced earlier types such as the sponsor and the fidepromissor. Researchers encountering fidejussor in Scots law or civil law materials should not assume it is identical to adpromissor without examining the specific procedural context.
ADPROMISSOR vs. CAUTIONER: In Scots law, the cautioner performs a functionally similar role to the adpromissor, and Black's equates them. Cautioner is the operative term in Scottish legal instruments and court records; adpromissor appears primarily in doctrinal and academic writing tracing Scots law back to Roman sources. Researchers working in Scottish archives should expect cautioner, not adpromissor, in transactional documents.
Why It Matters in Research
This term is a marker term, not a working term. It will rarely appear in contracts, court pleadings, or transactional records; it appears in doctrinal treatises, civil law commentaries, Scots law scholarship, and legal dictionaries tracing the genealogy of suretyship doctrine. If you encounter adpromissor in a primary source, it is almost certainly a learned or academic text engaging with Roman law foundations.
Researchers working in Scots law should treat adpromissor as a gateway term pointing to the cautionary obligation system. The Scots law of caution draws heavily on Roman fidejussion, and understanding the adpromissor's role clarifies why Scots cautionary doctrine diverges from English guarantee law in certain structural ways — particularly on the question of whether the surety's obligation is truly accessory or independently constituted.
In comparative law research, the adpromissor/fidejussor lineage is essential context for tracing how modern civil law systems in continental Europe and Louisiana handle guaranty obligations differently from common law systems. The civilian insistence on the accessory character of the surety's promise — the adpromissor adds to the principal's promise, he does not substitute for it — continues to shape civilian guaranty doctrine today.
Trap for researchers: Black's first and second editions give nearly identical definitions. Do not assume the second edition updated or refined the entry. The Burrill entry is the more analytically useful of the three source definitions because it identifies the parallel performance structure ("engages to do the same thing that the promissor does") and cites Digest 45.1.52, which grounds the term in a traceable primary source of Roman law.
Historical Dictionary Support
All three dictionary sources agree on the core definition: the adpromissor is a surety who adds his own promise to the principal debtor's promise. Black's (both editions) and Burrill converge on this without meaningful divergence. Burrill's entry is marginally superior for research purposes: it provides the Digest citation (Dig. 45.1.52), which connects the term to Roman law's treatment of stipulatory obligations, and it makes explicit that the adpromissor engages to do the same thing as the principal — clarifying the parallel rather than merely accessory structure of the obligation.
What the historical dictionaries do not address is the internal taxonomy of Roman accessory obligors. Roman law recognized at least three types of adpromissors — the sponsor, the fidepromissor, and the fidejussor — each with different rules regarding heritability of the obligation, benefit of division, and applicability to the obligations of non-citizens. The dictionaries collapse this into a single definition, which is sufficient for most legal research purposes but inadequate for Roman law scholarship or civil law comparative work.
Jurisdictional Note
The adpromissor as a functional legal category is relevant primarily in civil law jurisdictions and in Scots law. It has no operative role in English common law, where suretyship developed through different doctrinal channels. In Louisiana, which inherited civilian suretyship doctrine, the structural concepts underlying the adpromissor inform the law of suretyship in the Louisiana Civil Code, though the term itself does not appear in modern Louisiana statutes.