Definition
Res accessoria is a Latin phrase from the civil law meaning an accessory thing — a thing that belongs to, is connected with, or depends upon a principal thing. The concept encodes a foundational rule of property and obligation: the accessory follows the principal. This rule is expressed in the maxim res accessoria sequitur rem principalem, meaning that the legal fate of the accessory thing is governed by the legal fate of the thing to which it is attached.
In practice, the doctrine means that when ownership, transfer, encumbrance, or destruction applies to the principal thing, it applies equally to whatever is accessory to it — unless parties have agreed otherwise or law provides an exception. A fixture attached to land, a clause ancillary to a primary contract obligation, or a pledge given as security for a debt each illustrates the relationship: the accessory cannot survive independently of the principal in the same legal sense.
The term is closely related to, and sometimes used interchangeably with, accessio — the civil law doctrine governing accession of property.
Common Confusion
Res accessoria should not be confused with the general English word "accessory" in either its criminal law sense (a person who aids a principal offender) or its colloquial sense (a supplemental item). The legal doctrine concerns things (res), not persons, and operates in property and civil obligation contexts. Researchers encountering "accessory" in criminal law sources are in an entirely different doctrinal neighborhood.
The term should also be distinguished from res communes (things common to all, such as air and running water), which appears immediately adjacent to res accessoria in several historical dictionaries and addresses the nature of ownership rather than the relationship between a principal and dependent thing.
Why It Matters in Research
Res accessoria functions primarily as a principle-carrying term rather than an independent cause of action or pleading. Researchers are most likely to encounter it in three contexts:
First, in civil law jurisdictions and mixed-law systems (Louisiana, Quebec, South Africa, Scotland), the doctrine is operative and referenced by name in property and contract analysis. Common law jurisdictions absorbed the underlying concept without always preserving the Latin label, so a researcher in English or American sources may find the idea argued under "fixtures," "appurtenant rights," "accession," or "principal and incident" without the phrase res accessoria appearing at all.
Second, in historical sources — particularly treatises and digests drawing on Roman law or civil law commentators — the phrase appears as a term of art requiring understanding of the broader accessio doctrine. Burrill's reference to Mackeldey's Civil Law (§§ 152–153) and Black's and Rapalje's shared citation to Broom's Legal Maxims are the key connective threads. Researchers who encounter res accessoria in a nineteenth-century American treatise should check whether the author is reasoning from civil law analogy or asserting it as operative doctrine in a common law jurisdiction — these are very different things.
Third, the maxim itself (res accessoria sequitur rem principalem) appears in maxim collections and is sometimes invoked as persuasive authority in equity and property disputes even in common law courts. Broom's Legal Maxims is the primary vehicle for this usage in Anglo-American sources.
A navigational trap: indexing in older digests may place this term under "Res" headings alongside dozens of other res phrases (res communes, res gestae, res ipsa loquitur, res judicata, etc.). These share only the Latin word for "thing" and are otherwise unrelated. Do not assume doctrinal kinship among res phrases based on proximity in a digest index.
Historical Dictionary Support
Black's, Rapalje & Lawrence, and Burrill are in close agreement on the core definition: an accessory thing belonging to or connected with a principal thing, governed by the maxim that the accessory follows the principal. All three cite Broom's Legal Maxims as authority for the maxim itself, and Burrill adds the primary civil law source — Mackeldey's Civil Law, §§ 152–153 — identifying accessio as the alternate term.
Rapalje & Lawrence note that res accessoria is "something belonging to, or connected with, the principal thing," which tracks the Roman law concept accurately. None of the three historical dictionaries elaborate on the practical scope of the doctrine or its application in common law contexts, reflecting the term's status as a civil law importation rather than a term of art developed through Anglo-American case law.
What the historical dictionaries miss: they do not address the tension between this civil law principle and common law property doctrines that reach similar results through different analytical frameworks (e.g., the fixture analysis in real property, or the "follows the principal" logic in suretyship and security interests). Researchers working in nineteenth-century American law should be alert to this translation gap.
Jurisdictional Note
The doctrine is most operative in civil law and mixed jurisdictions. In American common law states, the underlying principle — that accessories follow the principal — is absorbed into specific doctrines (fixtures, appurtenances, accessory obligations) without routine use of the Latin phrase. Louisiana, as a civil law jurisdiction, applies the concept more directly and by name in its property code framework.