Editing Letter case (section)

===Unit symbols and prefixes in the metric system===
[[File:SI base units.svg|thumb|Of the seven [[SI base unit|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 (unit)|mole]] for [[amount of substance]]) are written in lower case.]]

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 in lower case.<ref name="SI brochure" /> For example:

* 1&nbsp;s (one&nbsp;[[second]]) when used for the [[SI base unit|base unit]] of [[time]].
* 1&nbsp;S (one&nbsp;[[siemens (unit)|siemens]]) when used for the unit of [[electric conductance]] and [[admittance]] (named after [[Werner von Siemens]]).
* 1&nbsp;Sv (one&nbsp;[[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.<ref name="SI brochure" /> For example, "one&nbsp;litre" may be written as:

* {{not a typo|1 l}}, the original form, for typefaces in which "digit one" {{angbr|1}}, "lower-case ell" {{angbr|l}}, and "upper-case i" {{angbr|I}} look different.
* 1&nbsp;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 "[[ℓ|script l]]" in various typefaces (e.g.: 1&nbsp;<span style="font-family: cursive;">l</span>) has traditionally been used in some countries to prevent confusion; however, the separate [[Unicode character]] which represents this, {{Unichar|2113|SCRIPT SMALL L}}, is deprecated by the [[International System of Units|SI]].<ref>{{cite web|url=https://www.unicode.org/charts/beta/nameslist/n_2100.html|title=Letterlike symbols|work=Charts (Beta)|publisher=[[Unicode Consortium]]|access-date=28 July 2017}}</ref> Another solution sometimes seen in [[Web typography]] is to use a serif font for "lower-case ell" in otherwise sans-serif material (1&nbsp;<span style="font-family: serif;">l</span>).

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-|kilo]], meaning 10<sup>3</sup> = 1000 multiplier), whereas upper case is used for larger multipliers:<ref name="SI brochure" />

* 1&nbsp;mW, [[watt|milliwatt]], a small measure of [[power (physics)|power]] ("m" for [[milli-|milli]], meaning 10<sup>−3</sup> = 1/1000 multiplier).
* 1&nbsp;MW, megawatt, a large measure of power ("M" for [[mega-|mega]], meaning 10<sup>6</sup> = 1&nbsp;000&nbsp;000 multiplier).
* 1&nbsp;mS, [[millisiemens]], a small measure of electric conductance.
* 1&nbsp;MS, megasiemens, a large measure of electric conductance.
* 1&nbsp;mm, millimetre, a small measure of [[length]].
* 1&nbsp;Mm, megametre, a large measure of length.