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 simarly 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 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.[1] 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.
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 superior 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. The actual calibre of the round would differ as the actual value of the inch would vary from country to country.