page_title: Letter case
section_title: Unit symbols in the metric system
hierarchical_section_title: Letter case / Stylistic or specialised usage / Unit symbols in the metric system
context_page_description: Letter case is the distinction between the letters that are in larger upper case and smaller lower case in the written representation of certain languages. The writing systems that distinguish between the upper and lower case have two parallel sets of letters, with each letter in one set usually having an equivalent in the other set. The two case variants are alternative representations of the same letter: they have the same name and pronunciation and are treated identically when sorting in alphabetical order.
  Letter case is generally applied in a mixed-case fashion, with both upper- and lower-case letters appearing in a given piece of text for legibility. The choice of case is often prescribed by the grammar of a language or by the conventions of a particular discipline. In orthography, the upper case is primarily reserved for special purposes, such as the first letter of a sentence or of a proper noun, which makes the lower case the more common variant in regular text.
  In some contexts, it is conventional to use one case only.
context_section_description: In the International System of Units (SI), a letter usually has different meanings in upper and lower case when used as a unit symbol. Generally, unit symbols are written in lower case, but if the name of the unit is derived from a proper noun, the first letter of the symbol is capitalised (nevertheless, the name of the unit, if spelled out, is always considered a common noun and written accordingly):
  1 s (one second) when used for the base unit of time.
  1 S (one siemens) when used for the unit of electric conductance and admittance (named after Werner von Siemens).
  1 Sv (one sievert), used for the unit of ionising radiation dose (named after Rolf Maximilian Sievert).
  For the purpose of clarity, the symbol for litre can optionally be written in upper case even though the name is not derived from a proper noun:
  1 l, the original form, for typefaces in which "digit one" ⟨1⟩, "lower-case ell" ⟨l⟩, and "upper-case eye" ⟨I⟩ look different.
  1 L, an alternative form, for typefaces in which these characters are difficult to distinguish, or the typeface the reader will be using is unknown. A "script l" in various typefaces (e.g.: 1 l) has traditionally been used in some countries to prevent confusion; however, the separate Unicode character which represents this, U+2113 ℓ SCRIPT SMALL L, is deprecated by the SI.  Another solution sometimes seen in Web typography is to use a serif font for "lower-case ell" in otherwise sans-serif material (1 l).
  The letter case of a prefix symbol is determined independently of the unit symbol to which it is attached. Lower case is used for all submultiple prefix symbols and the small multiple prefix symbols up to "k" (for kilo, meaning 10³ = 1000 multiplier), whereas upper case is used for larger multipliers:
  1 ms, a small measure of time ("m" for milli, meaning 10⁻³ = 1/1000 multiplier).
  1 Ms, a large measure of time ("M" for mega, meaning 10⁶ = 1 000 000 multiplier).
  1 mS, a small measure (in millisiemens) of electric conductance.
  1 MS, a large measure (megasiemens) of electric conductance.
  1 mm, a small measure of length (the latter "m" for metre).
  1 Mm, a large measure of length.
caption: Of the seven SI base-unit symbols, "A" (ampere for electric current) and "K" (kelvin for temperature), both named after people, are always written in upper case, whereas "s" (second for time), "m" (metre for length), "kg" (kilogram for mass), "cd" (candela for luminous intensity), and "mol" (mole for amount of substance) are written in lower case.