The line (abbreviated L or l) was a small English unit of length, variously reckoned as 1⁄10, 1⁄12, 1⁄16, or 1⁄40 of an inch. It was not included among the units authorized as the British Imperial system in 1824.
The line was not recognized by any statute of the English Parliament but was usually understood as 1⁄4 of a barleycorn, which itself was recognized by statute as 1⁄3 of an inch but often reckoned as 1⁄4 of an inch instead. The line was eventually decimalized as 1⁄10 of an inch, without recourse to barleycorns. The button trade used the term, redefined as 1⁄40 of an inch.
Botanists formerly used the units (usually as 1⁄12 inch) to measure the size of plant parts. Linnaeus's Philosophia botanica (1751) includes the Linea in its summary of units of measurements, defining it as "Linea una Mensurae parisinae"; Stearns gives its length as 2.25 mm. Even after metrication, British botanists continued to employ tools with gradations marked as linea (lines); the British line is approx. 2.1 mm and the Paris line approx. 2.3 mm.
Gunsmiths and armament companies also employed the line, in part owing to the importance of the German and Russian arms industries. These are now given in terms of millimeters, but the seemingly arbitrary 7.62 mm caliber was originally understood as a 3-line caliber (as with the 1891 Mosin–Nagant rifle). The 12.7 mm caliber used by the M2 Browning machine gun was similarly a 5-line caliber.
Other similar small units called lines include:
- The Russian liniya, 1⁄10 of the diuym which had been set precisely equal to an English inch by Peter the Great
- The French ligne (lig.) or Paris line, 1⁄12 of the French inch (pouce) and about 1.06 L.
- The Portuguese linha, 1⁄12 of the Portuguese inch or 12 "points" (pontos) or 2.29 mm
- The German linie was usually 1⁄12 of the German inch but sometimes also 1⁄10 German inch
- English units used prior to 1824
- Imperial units defined by the British Weights and Measures Act of 1824
- Ligne, the equivalent French unit
- Cardarelli, F. (2004), Encyclopaedia of Scientific Units, Weights and Measures: Their SI Equivalences and Origins, 2nd ed., Springer, ISBN 1-85233-682-X.
- Cole, Rory Ely (2002), Common Linear Measure (Years of 732 & 1154).
- Hogg, Ian V.; et al. (1991), Military Small Arms of the 20th Century, 6th ed., Guild Publishing.
- Jefferson, Thomas (4 July 1790), Report on the Subject of Measures, Weights, and Coins, New York.
- Niles, Hezekiah, ed. (1814), "Jefferson on Weights and Measures: Letter from the Secretary of State to the Speaker of the House of Representatives: New-York, July 4, 1790", The Weekly Register, Vol. V (Sept. 1813 – Mar. 1814), Baltimore: Franklin Press, pp. 20–26.