Definition
A unit of weight historically recognized in law. Two distinct standards apply depending on context:
1. Troy ounce: One-twelfth of a troy pound, used in the weighing of precious metals, gold, silver, and pharmaceuticals. The troy system divides the pound into 12 ounces of 480 grains each.
2. Avoirdupois ounce: One-sixteenth of an avoirdupois pound, used in general commerce and trade for ordinary goods. The avoirdupois ounce contains 437.5 grains.
In legal contexts — particularly statutes governing weights and measures, trade regulations, drug quantity thresholds in criminal law, and commodity standards — the applicable system must be determined by subject matter. Precious metals default to troy; most other goods default to avoirdupois.
Common Language
Modern common usage (Wiktionary): Primarily denotes the avoirdupois unit in everyday American English. Wiktionary also lists a secondary zoological sense — a large wild feline, including the snow leopard (Panthera uncia) — sharing the same spelling but entirely unrelated in origin or legal significance.
Historical common usage (Webster's 1913): Defined as the sixteenth part of an avoirdupois pound (437.5 grains), with the troy ounce as a secondary definition. Also noted a now-obsolete figurative sense meaning "a small portion."
Editorial note: The common speaker uses "ounce" to mean the avoirdupois unit without further thought. Legal instruments, however, must be read in light of which weight system governs — a distinction that can affect the outcome in criminal drug prosecutions (where quantity triggers mandatory minimums), commodity contracts, and statutory compliance. The gap is narrow but consequential when precision is at stake.
Common Confusion
Troy vs. avoirdupois: The troy ounce (480 grains) is heavier than the avoirdupois ounce (437.5 grains). An instrument that specifies "one ounce" without qualification leaves room for dispute in any context where the two systems overlap. Historically, the legal definition leaned toward troy — reflecting the classical Roman and medieval English inheritance of the pound as twelve ounces — but modern statutory and commercial practice defaults to avoirdupois except in precious metals and pharmacy. Researchers reading older statutes, contracts, or weights-and-measures ordinances should not assume the modern avoirdupois default applies.
Why It Matters in Research
The primary research risk with "ounce" is anachronistic reading. Older statutes and legal instruments written in the troy tradition defined one pound as twelve ounces. Modern readers, trained to think in avoirdupois sixteenths, may misread quantities in historical texts. This matters in:
— Criminal law: Federal and state drug statutes specify quantity thresholds in grams or ounces. Prosecutors and defense counsel must confirm the applicable system. Most modern drug statutes specify avoirdupois weight, and courts have generally applied that standard, but the issue has arisen in litigation.
— Commercial law and commodity contracts: Contracts for gold, silver, and pharmaceutical ingredients typically specify troy weight. General goods contracts run avoirdupois. An unqualified "ounce" in a historical commercial instrument requires extrinsic examination of trade custom.
— Weights and measures statutes: Federal and state weights-and-measures law has evolved through multiple eras. Researchers tracing compliance obligations across time must track which statutory system was in force.
— The Bouvier entry is conspicuously incomplete — "See" with no following reference — suggesting either a printing error or a cross-reference that did not survive in the digitized corpus. Researchers relying solely on Bouvier for this term will find no useful content.
Historical Dictionary Support
The historical sources are in modest agreement but reveal meaningful evolution. Black's first edition gives only the troy definition — "the twelfth part of a pound" — reflecting the classical inheritance of the term. Black's second edition corrects and expands this, acknowledging both the troy twelfth and the avoirdupois sixteenth, which signals the editors' recognition that by the early twentieth century both systems were legally operative and the reader needed both.
Burrill follows the classical line, citing the Latin uncia and emphasizing "the twelfth part of any thing" — consistent with Roman law's use of uncia as a general fractional unit (one-twelfth) applicable beyond weight to inheritance shares and property divisions. This broader Roman sense is worth noting: in classical and early common law sources, uncia could denote a fraction generally, not merely a weight. Researchers in Roman-influenced civil law jurisdictions or reading early American legal documents influenced by civilian tradition should keep this in mind.
Bouvier adds nothing beyond the label "a weight" and a truncated cross-reference.
Webster's 1913 aligns with the avoirdupois-primary, troy-secondary ordering that was becoming standard in American commercial life by that period.
No historical source addresses the modern criminal law application — mandatory minimum drug quantities — because that use is a twentieth- and twenty-first-century development.
Jurisdictional Note
Federal drug statutes and the U.S. Pharmacopeia use avoirdupois weight as the default for most substances. Precious metals trading in the United States and internationally runs on troy weight. State weights-and-measures statutes largely track the federal avoirdupois standard for commerce. Researchers working in civil law jurisdictions with a Roman law inheritance may encounter uncia in its broader fractional sense in older materials.