Definition
A ton; a unit of measure used in medieval English law, particularly in the context of customs, duties, and trade regulation. The term appears in Latin-language legal instruments and administrative records of the feudal and early common law period, functioning as the Latinized equivalent of the English "ton" or "tun." In practice, tonna referred most often to a large cask or barrel of a standardized capacity — originally associated with wine — which became the basis for calculating cargo weight, port dues, and related commercial obligations.
Common Language
Modern common usage (Wiktionary): "Ton" — a unit of weight equal to 2,000 pounds (short ton, U.S.) or 2,240 pounds (long ton, U.K.), or a metric ton of 1,000 kilograms. Also used informally to mean a large quantity.
Historical common usage (Webster's 1913): "Ton" — a measure of weight, commonly 2,240 pounds avoirdupois in the United States and England; also, a measure of internal capacity for ships, equal to 100 cubic feet.
The gap between the modern common meaning and the historical legal meaning of tonna is significant. In medieval legal usage, tonna was not primarily a unit of weight but a unit tied to a specific physical object — the tun cask — with capacity measured in volume. The legal and commercial obligations calculated against it (such as tunnage or poundage duties) derived from that volumetric, vessel-based standard, not from the abstract weight-based scale familiar today.
Recognized Forms
/SUBTYPES
Tonna appears as part of a cluster of related medieval revenue and trade terms. The most consequential derivative is:
Tunnage (also tonnage): The customs duty levied on each tun of wine or equivalent cargo unit imported into England. This became a significant parliamentary revenue grant to the Crown, with tunnage and poundage forming the combined customs grant that was a recurring subject of constitutional dispute in English history.
Why It Matters in Research
Tonna is a term a researcher will encounter in Latin-language primary sources — writs, customs records, port books, and royal grants — rather than in reported cases or modern statutes. Its significance is almost entirely archival and historical.
Key navigational points:
First, do not conflate tonna (the unit or cask) with the later legal concept of tonnage as a tax or duty. The term tonna names the measure; tunnagium or tunnage names the levy assessed against it. Sources sometimes blur this distinction, particularly in secondary commentary.
Second, the standardization of the tun cask was itself variable across time and commodity. A tun of wine differed in capacity from a tun of honey or oil in some periods. Researchers working with customs accounts or cargo disputes in medieval sources should not assume a single fixed measure.
Third, tonna connects directly to the development of English admiralty and maritime law. Duties calculated in tonna-based units appear in early admiralty jurisdiction materials and in the history of tonnage and poundage, which carries forward into seventeenth-century constitutional conflicts between Parliament and the Crown over the lawful grant of customs revenues.
Fourth, the term's appearance in legal instruments signals a Latin-language document of significant age. If tonna appears in a source presented as more recent than the early modern period, treat that as a provenance flag warranting scrutiny.
Historical Dictionary Support
Burrill's Law Dictionary provides the entry concisely: "L. Lat. In old English law. A ton. Spelman." The citation to Spelman refers to Sir Henry Spelman's Glossarium Archaiologicum, the foundational glossary of medieval Latin legal and administrative terms in the English tradition. Spelman's work remains the primary scholarly reference for this class of archaic Latin legal terminology.
Burrill's entry is accurate but minimal. It confirms the core meaning and its limitation to old English law but does not develop the commercial or fiscal context in which tonna actually functioned. Researchers relying solely on Burrill's entry may underestimate the term's practical importance in medieval customs and trade law.
No significant divergence exists among historical dictionaries on this term — it is narrow, consistent, and well-confined to its archaic period.
Jurisdictional Note
Tonna is specific to English legal history. It does not carry operative legal meaning in any modern common law jurisdiction. Researchers working in Scottish, Irish, or continental European medieval sources may encounter analogous Latin terms for barrel-based measures, but these are distinct lexical traditions and should not be conflated with the English tonna without independent verification.