The line is a unit of measurement, one line being equal to 1⁄12 of an English (prior to 1824) inch. It was defined as one-quarter of a barleycorn, which defined the inch even before 1066. The French (prior to 1799) ligne was similarly defined as 1⁄12 of the pouce (meaning "thumb", about an inch). Since the French pouce or inch was about 6% longer than the English inch, the "line" was similarly longer. The Portuguese linha was defined in the end of the 15th century as 1⁄12 polegada (Portuguese inch of 27,5 mm). The linha in turn, was divided into 12 pontos (points). The Russian liniya was defined as 1⁄10 of the diuym (inch) from the 16th to the early 20th century; the diuym itself was redefined as exactly an English inch by Peter the Great. The German linie is described in the article German units of measurement as "usually 1⁄12 inch, but also 1⁄10". Since the Russian military and German manufacturing were major factors in arms procurement, the "1⁄10-inch line" became common terminology concerning weapons.
The button trade also used the term, but defined it as being 40 lines to the inch.  Botanists have used the unit to measure the size of plant parts and also defined it as being 12 lines to the inch.
In older botanical and zoological texts, it was a common customary unit, as in the term 'awns 3 to 4 lines long'. Even after the properties of the metric system were recognized for technical pursuits, existing tools frequently favored the use of customary units. Thus a 7.62 mm caliber round seems numerically arbitrary, until it is realised that 7.62 mm is 0.3 inches, .30 cal or three-lines. The 1891 Russian Mosin-Nagant rifle is known as the "three-line rifle" in Russian. Although rarely referred to as such, the 12.7 mm Browning HMG round is a "five-line" round.
- English units used prior to 1824
- Imperial units defined by the British Weights and Measures Act of 1824
- Ligne based on the French pouce
- Obsolete Russian units of measurement
- Portuguese customary units
- Fátima Paixão, Fátima Regina Jorge, Success and constraints in adoption of the metric system in Portugal, The Global and the Local: History of Science and the Cultural Integration of Europe, 2006
- Cardarelli, F. (2004). Encyclopaedia of Scientific Units, Weights and Measures: Their SI Equivalences and Origins (2nd ed.). Springer. pp. 121–124. ISBN 1-85233-682-X.
- Military small arms of the 20th century, 6th edition, Ian V Hogg and John Weeks, Guild Publishing, 1991.