Definition
A system of weights in which the pound contains sixteen ounces and the ounce contains sixteen drams. Avoirdupois is the standard weight system historically applied to common commercial goods — grain, hay, butter, sugar, coal, and similar bulk commodities — as distinguished from the troy system (used for precious metals and gems) and the apothecaries' system (used for medicinal substances). In legal contexts, the term appears most often in statutes and contracts governing the sale or taxation of goods by weight, where specifying the applicable weight system was necessary to determine the quantity of goods exchanged or assessed.
Common Language
Modern common usage (Wiktionary): The official system of weights used in the UK (1856–1963) and the USA (1866–1959), with historical customary use in London since 1300. Colloquially, also used to mean weight or heaviness generally, as in a person's body weight.
Historical common usage (Webster's 1913): "Goods sold by weight" (obs.); a system for weighing coarser commodities such as hay, grain, butter, sugar, and tea. Also used colloquially for bodily weight or heaviness.
The gap between common and legal meaning is narrow but worth noting: in ordinary speech, avoirdupois drifted toward a humorous reference to a person's physical bulk. In legal and commercial contexts, the word carries a precise technical meaning — a defined and legislatively recognized system of weights — and its appearance in a contract or statute is a term of art, not a casual reference to heaviness.
Why It Matters in Research
Researchers encounter avoirdupois primarily in three contexts: weights-and-measures statutes, commercial contracts for the sale of goods by weight, and customs and tariff records.
The critical research trap is the three-system problem. Historical sources — particularly contracts, court records, and statutes predating the twentieth century — often do not specify which weight system governs. A pound avoirdupois (7,000 grains) differs materially from a troy pound (5,760 grains). Disputes over short weight, fraud in the sale of goods, and customs duty assessments frequently turned on which system applied. When reading historical commercial litigation, never assume the weight system without confirming it from the document or the commodity involved.
Jurisdictional and statutory variation matters here. U.S. federal weights-and-measures law, British imperial statutes, and individual state or colonial regulations each defined avoirdupois weights with slight variations at different periods. When dating a document, the applicable statutory definition of the pound or ounce may differ from modern equivalents.
Anderson's cross-reference to TON is practically important: the legal definition of a ton — whether a short ton (2,000 lb avoirdupois), long ton (2,240 lb avoirdupois), or metric ton — derives directly from the avoirdupois system, and disputes over tonnage in shipping, mining, and commodity contracts are downstream of how avoirdupois was defined at the operative moment.
For researchers working in the Law Mind corpus, avoirdupois most often appears as a silent assumption rather than an explicit term. Its absence from a document does not mean the system was not in use; it typically means it was so standard for the commodity in question that no specification was thought necessary.
Historical Dictionary Support
The historical sources agree completely on the core definition: sixteen ounces to the pound, applied to common goods rather than medicines, metals, or precious stones. Black's first and second editions are nearly verbatim, the second containing only a minor typographical variant ("metuls" for "metals"). Neither edition goes further than the bare definition.
Anderson's Dictionary of Law adds nothing substantive, directing the reader instead to the entry for TON — a pragmatic choice reflecting where avoirdupois most commonly generated legal disputes. This cross-reference suggests that by the time Anderson compiled his dictionary, the definitional content of avoirdupois was settled; the live legal questions were downstream, in the derived units.
What the historical dictionaries collectively miss is the statutory complexity: the precise legal weight of an avoirdupois pound varied slightly across jurisdictions and periods, and the question of which legislative definition controlled a given transaction was not trivial. The dictionaries treat the system as uniform, which was broadly true in practice but not always true in law.
Jurisdictional Note
The United States formally defined avoirdupois weights by federal statute, with the pound standardized relative to the kilogram under the Metric Act of 1866. The United Kingdom operated under a succession of Weights and Measures Acts. Researchers working across jurisdictions or across historical periods should verify the applicable statutory definition, particularly for pre-standardization documents where local custom may have governed.