Definition
A Latin adjective from medieval and early modern English legal usage meaning "reasonable." The term appears in legal instruments, writs, and maxims as a modifier indicating that a thing — a time period, a portion, a custom, or an act — must meet the standard of reasonableness rather than conforming to a fixed rule. It does not import a precise measurement but instead invokes judicial discretion to assess what is adequate or fitting under the circumstances.
The term appears most prominently in compounds and Latin writs of the old common law, particularly in phrases like rationabile tempus (a reasonable time), rationabilis pars (a reasonable part or share), and as a component of formal writs such as de rationabili parte bonorum, which concerned a widow's or child's reasonable share of a deceased's personal estate. In each context, the word signals that the governing standard is proportionality and fairness as assessed by law, rather than a fixed statutory measure.
Why It Matters in Research
Researchers encountering rationabilis in historical sources must recognize that it operates as a legal placeholder — a signal that the question was left to judicial determination rather than resolved by rule. This makes it both flexible and elusive: you cannot find a fixed definition because none existed. The standard shifted with context, period, and the discretion of particular justices.
The term is concentrated in pre-modern English sources — Year Books, early common law pleadings, Bracton, Coke's commentary, and the writs literature. It rarely survives into post-1800 legal writing in its Latin form, having been absorbed into the English word "reasonable" and its surrounding doctrine. A researcher who encounters rationabilis in a historical instrument should treat it as a substantive legal term of art, not merely decorative Latin, and look for the specific writ or cause of action it qualified.
The writ de rationabili parte (sometimes de rationabili parte bonorum) is the most practically significant application. It was a writ of right for a widow or children to recover their reasonable share of an intestate's personal estate — a claim that in some jurisdictions merged into statutory administration schemes, in others was abolished outright. Understanding that rationabilis anchored this cause of action helps researchers trace the doctrine through the writ literature and into early equity and probate practice.
Pay attention to the distinction between rationabilis as a term qualifying time (rationabile tempus) versus one qualifying entitlement (rationabilis pars). These appear in different procedural contexts and draw on different bodies of precedent.
Historical Dictionary Support
Burrill's Law Dictionary provides the essential baseline: rationabilis means reasonable, derived from ratio, and the term's indeterminacy was acknowledged explicitly in medieval legal writing. Burrill quotes Coke's commentary on Littleton — Quam longum esse debet rationabile tempus non definitur in lege, sed pendet ex discretione justitiariorum — which translates as: how long a reasonable time ought to be is not defined in law, but rests in the discretion of the justices. This passage from Co. Litt. 56b is among the clearest early articulations of what rationabilis means procedurally: not a rule, but a standard.
Burrill's cross-references to de rationabili and de rationabilibus point toward the writ literature, where rationabilis does its heaviest lifting. Historical dictionaries of English law generally treat the term in passing rather than as a primary entry, folding its meaning into discussions of specific writs or the general doctrine of reasonableness. This reflects the term's character: it is connective tissue rather than a stand-alone concept.
What historical sources tend to understate is how rationabilis participated in the larger common law structure of standards versus rules — a distinction that modern jurisprudence has developed extensively but that was already embedded in medieval Latin legal usage. Researchers working on the intellectual history of the reasonableness standard will find rationabilis a useful early data point.