Definition
A Latin legal term with two related but distinct applications in the historical common law and civil law traditions:
1. (Civil law) An accessory or incident thing — something that belongs to or depends upon something else. Used adjectivally in phrases such as res accessoria (an accessory thing) to describe property or rights that are subordinate to and follow a principal thing.
2. (Old English and common law) A person who is an accessory or accessary — one who participates in a criminal act not as the principal offender but in a secondary or assisting capacity. In this sense, accessorius names the legal status of the participant, not merely a descriptive quality.
The term is most frequently encountered in legal literature through the maxim: Accessorius sequitur naturam sui principalis — an accessory follows the nature of his principal. The maxim carries substantive legal force: an accessory cannot be convicted of a higher degree of crime than that of which the principal is convicted, and the accessory's liability tracks the principal's in both character and grade.
Common Confusion
Accessorius (the Latin form) should not be confused with the English-language terms accessary and accessory, which are themselves frequently conflated in historical sources. Accessary was the dominant English legal spelling through the nineteenth century, particularly in criminal law, while accessory became common both in law and ordinary usage. Burrill uses accessary; Anderson uses accessary and assistant interchangeably. A researcher encountering accessorius in a Latin legal text should confirm whether the context is criminal liability (the common law sense) or property and civil law (the civil law sense), as the two uses are distinct.
Why It Matters in Research
Accessorius appears almost exclusively in Latin legal maxims, civil law treatises, and older English criminal law materials. Researchers are unlikely to encounter it as a standalone operative term in modern sources, but it surfaces frequently in:
— Maxim collections and digests, where the phrase Accessorius sequitur naturam sui principalis is quoted and applied. The maxim remained influential well into the nineteenth century in common law jurisdictions and shaped doctrine around the grade of criminal liability for accessories.
— Civil law materials, where res accessoria and related phrases governed questions of property classification and the treatment of incidents, appurtenances, and subordinate rights. Here the term maps onto a broader civil law principle (accessorium sequitur principale) rather than criminal liability.
— Historical criminal law sources, including Coke's Institutes (Burrill cites 3 Inst. 139 for the maxim), where the distinction between principal and accessorius was structurally important before modern statutory abolition of most principal/accessory distinctions at the guilt stage.
A key research trap: the civil law use (a thing that is accessory to another thing) and the criminal law use (a person who is an accessory) share the same Latin root and the same term but operate in entirely different doctrinal frameworks. Cross-contamination between these two senses is possible in older digests and encyclopedias that index them together.
The maxim's continued citation in nineteenth-century American and English cases means that researchers tracing accessory liability doctrine should look for accessorius as an index term in Latin maxim tables, even when the surrounding discussion uses the English term accessary or accessory.
Historical Dictionary Support
Burrill and Anderson agree on the core meaning but differ in emphasis. Burrill is more careful to separate the civil law property sense (res accessoria, citing Mackeldey's Civil Law) from the criminal law sense, treating them as distinct entries. Anderson collapses them, defining accessorius primarily as a person — an assistant — and immediately pivoting to the maxim and criminal law consequences.
Both sources quote or reference the same maxim, but Anderson's rendering is slightly fuller on the substantive rule: an accessary follows the nature of his principal in treason and misdemeanors, and he cannot be guilty of a higher degree of crime. This formulation is significant because it reflects the historical limitation of the maxim's operation — treason and misdemeanor categories had distinct accessory rules at common law, and the maxim's application to felony was somewhat different in its procedural consequences.
Neither source develops the civil law property use beyond citation to Mackeldey, which is appropriate given that the term's primary doctrinal weight in common law sources was always the criminal liability maxim. Researchers needing the civil law property sense in depth should consult Mackeldey directly or continental civil law sources, as neither Burrill nor Anderson treats it as more than a cross-reference.
Jurisdictional Note
The criminal law maxim embedded in accessorius — that an accessory cannot be convicted of a higher degree of crime than the principal — was a common law rule of broad application in England and the United States, but its practical importance diminished as legislatures codified accessory liability and abolished the old procedural distinctions between accessories before and after the fact. Modern American statutes typically render the distinction a matter of sentencing discretion rather than a ceiling on criminal grade.