Definition
Si prius (Latin: "if before") is a term from old English practice referring to the formal words contained in ancient writs for summoning juries. The phrase appeared in the writ as a conditional clause directing the sheriff to act "if before" a specified time or event. It is most closely understood in context with its more familiar cousin, nisi prius ("unless before"), which became the dominant term in common law procedure for describing jury trial proceedings conducted in the counties before a judge on circuit. Si prius itself did not survive into modern procedural vocabulary; it remains a term of purely historical and philological interest, encountered in old writs and the medieval treatise Fleta.
Common Confusion
Si prius is frequently encountered alongside — and sometimes confused with — nisi prius, and the two share the same basic procedural DNA. Both appear in the formula of writs directing the summoning of jurors and both operate as conditional clauses. The critical distinction is functional and historical: nisi prius became the operative term of art for the entire system of circuit jury trials in English common law and later American practice, while si prius remained a narrower formalism that did not generalize beyond its original writ context. Researchers who encounter si prius in a source index or writ transcript should not assume the document concerns the nisi prius system broadly; the term marks a specific verbal formula, not a court or procedural system.
Why It Matters in Research
Si prius is essentially a dead-end term in substantive legal research — no doctrine, rule, or right turns on it — but it matters in two narrow contexts.
First, paleographic and archival research. Researchers working with original medieval or early modern English writs, particularly those related to jury assembly, may encounter si prius as a literal phrase in the document. Recognizing it prevents misreading the text and distinguishes this conditional clause from other Latin conditionals in writ formulary.
Second, source navigation. The term appears in historical legal dictionaries almost exclusively as a brief entry directing readers toward nisi prius, with Fleta (lib. 2, c. 65, § 12) cited as the primary authority. Any research that bottoms out at si prius in a secondary source is being pointed toward the nisi prius system for substantive content. Follow that thread. The corpus connections run from si prius → nisi prius writ → the broader nisi prius court system → circuit court practice in both English and early American jurisdictions.
Third, there is a minor indexing trap in older digests and encyclopedia volumes that alphabetize Latin phrases loosely. Si prius may appear near si quis, si te fecerit securum, and the siquid maxims, none of which are related in substance. Researchers should not infer a doctrinal grouping from proximity in an alphabetical list.
Historical Dictionary Support
All five source dictionaries are in complete agreement on the definition: si prius means "if before" and designates formal words in old writs for summoning juries, citing Fleta, lib. 2, c. 65, § 12. There is no divergence among the sources on meaning, usage, or authority.
Burrill's Law Dictionary is the most useful of the five for research navigation because it explicitly cross-references the nisi prius writ entry, making the interpretive connection plain. Bouvier's is the most contextually generous, surrounding the entry with related writ terminology (including si te fecerit securum) that helps a researcher understand the formulaic environment in which si prius appeared.
What all historical sources share is brevity — none treats si prius as a term of substantive doctrinal weight. The consistent terseness of the entries across two centuries of legal dictionary compilation is itself informative: this term was never contested, never elaborated, and never generalized. Its meaning was fixed, its use was archaic, and its significance was always derivative of the nisi prius framework.
Encyclopedia Cross-Reference
See Law Mind Encyclopedia: Nisi Prius; Jury Writs and Summoning Practice.