Characteristics of a Standard Unit

A standard unit for any quantity should be defined and used globally.

It should have universal acceptance. It should be accepted by General Conference on Weights and Measures.

The standard units should be independent of each other.

Symbols do not have an appended period/full stop (.).

Symbols are written in upright (Roman) type (m for metres, s for seconds), so as to differentiate from the italic type used for variables (m for mass, s for displacement). By consensus of international standards bodies, this rule is applied independent of the font used for surrounding text.

Symbols for units are written in lower case, except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, so its symbol is written "Pa", whereas the unit itself is written "pascal". All symbols of prefixes larger than 103 (kilo) are also uppercase.

The one exception is the litre, whose original symbol "l" is unsuitably similar to the numeral "1" or the uppercase letter "i" (depending on the typeface used), at least in many English-speaking countries. The American National Institute of Standards and Technology recommends that "L" be used instead, a usage which is common in the US, Canada and Australia (but not elsewhere). This has been accepted as an alternative by the CGPM since 1979. The cursive l is occasionally seen, especially in Japan and Greece, but this is not currently recommended by any standards body. For more information, see litre.

The SI rule is that symbols of units are not pluralised, for example "25 kg" (not "25 kgs").

The American National Institute of Standards and Technology has defined guidelines for American users of the SI. These guidelines give guidance on pluralising unit names: the plural is formed by using normal English grammar rules, for example, "henries" is the plural of "henry". The units lux, hertz, and siemens are exceptions from this rule: They remain the same in singular and plural. Note that this rule applies only to the full names of units, not to their symbols.