Litre | one of the metric units of volume

The German unit name liter [ˈliːtɐ, also: ˈlɪtɐ] comes from French litre [from Middle French litron (a hollow measure), Middle Latin litra, Greek lítra = pound].

In principle, in the International System of Units (SI), unit symbols are only capitalised if the unit is named after a person; thus, the unit symbol for the litre is a lower-case l.

The capital L, which is mainly used in English-speaking countries, was exceptionally approved by the CGPM as an alternative unit symbol for the litre in order to avoid confusion of the letter l with the digit 1, which can occur in some fonts. ISO and IEC use only the original unit symbol l. IUPAC and DIN 1301-1 allow both notations.

Occasionally, a lowercase l in cursive (ℓ) is also used to distinguish the unit symbol from the numeral 1 or the uppercase letter I. The associated Unicode code point for the "ℓ" is U+2113. The unit name liter used to be neuter according to DIN 1301-1. However, this no longer coincided with common usage. The Duden, for example, mentions the neuter form only in second place. With DIN 1301-1:2010-10, this definition was changed from DIN 1301-1:2002-10; the unit name litre is now defined as masculine.

In 1793, the litre was introduced in France as the new republican unit of measurement (in connection with the metric system) and was equated with a cubic decimetre.

In 1879, the CIPM adopted the French liter definition and mandated the use of the l (lowercase l) as a symbol.

In 1901, the litre was redefined at the third CGPM to that volume which 1 kg of pure water has at the temperature of its highest density under normal pressure (1013.25 hPa). The liter was thus about 1.000 028 dm³ in size (originally 1.000 027 dm³ was given for the conversion). Due to the new task for this unit of measurement of coupling mass and volume via a special measurand of water, pure water under these conditions still had a density of 999.975 kg/m³, but now exactly 1.000 kg/l.

In 1964, at the 12th CGPM, the definition of 1793 was restored. Since then, a litre has again been exactly one cubic decimetre, and water has a density of 0.999 975 kg/l, as it did before 1901.

In 1979, at the 16th CGPM, the alternative symbol L (capital letter L) was approved for the litre; at the same time, the wish was expressed to keep only one of the two symbols (lower case or capital letter) in the future. Finally, in 1990, it could only be stated that it was still too early for a decision.