Definition
A unit of lineal measure equal to two and one-quarter inches. As a legal measure, a nail was used primarily in the textile trade to measure cloth, ribbon, and similar fabric goods. Sixteen nails equal one yard.
Common Language
Modern common usage (Wiktionary): A spike-shaped metal fastener used for joining wood or similar materials; also, the thin horny plate at the ends of human fingers and toes, or the claw of a bird or animal.
Historical common usage (Webster's 1913): The horny scale or plate of epidermis at the end of the fingers and toes; a slender, pointed piece of metal usually with a head, used for fastening pieces of wood or other material together.
The gap here is complete. Common usage — both modern and historical — gives nail as a fastener or anatomical feature. The legal and trade meaning is a unit of measurement with no intuitive connection to either common sense. A researcher encountering "nail" in a contract, invoice, or trade statute without this context would likely misread the document entirely.
Why It Matters in Research
The primary research risk is invisible misreading. A researcher working through historical commercial documents — fabric contracts, customs records, draper's accounts, textile trade statutes — may encounter "nail" as a unit of measure without any signal that it is being used in that sense. The word appears without a qualifier. Context must do the work.
This meaning was most active in English commercial and trade law during the period when textile measurement was heavily regulated. Parliamentary statutes governing cloth measurement used the nail alongside the yard, ell, and quarter. Researchers working in pre-twentieth-century English trade law, import and export records, or merchant disputes involving fabric goods should flag every appearance of "nail" as a potential unit of measure rather than a common object.
The term appears less frequently in American legal sources, where the ell system was less institutionalized, but it does appear in colonial-era and early republic commercial documents following English trade conventions.
Historical Dictionary Support
All three source dictionaries — Black's (both editions) and Bouvier — are in complete agreement: a nail is a lineal measure of two and one-quarter inches. The entries are unusually brief and uniform, suggesting the compilers treated this as settled and uncontested trade vocabulary requiring no elaboration. Bouvier cross-references the broader entry on MEASURE, which is the appropriate place to situate this term within the larger system of linear measurement.
What the historical dictionaries do not address: the trade context that gave this measurement its practical significance, the statutory history of cloth measurement in English law, or the relationship between the nail and closely adjacent units (the quarter, the ell, the yard). Researchers needing that context must look beyond the dictionary entries to trade law sources and historical weights-and-measures literature.
Jurisdictional Note
The nail as a unit of measure is predominantly an English law and trade concept. American usage followed English convention in colonial and early republic commercial practice but the term did not become embedded in American statutory measurement systems. Researchers working in English sources will encounter it more frequently than those working in American materials after independence.