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Rules for writing SI units

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As noted in Introduction to SI units there are specific rules about how SI units should be written. These include:

 

-          Values are written as a number followed by a space (representing a multiplication sign) and a unit symbol, e.g. 2.4 kg or  J. This convention also applies to the per cent sign (%). Exceptions are the symbols for plane angular degrees, minutes and seconds (°, ′ and ″), which are placed immediately after the number with no intervening space.

 

-          Symbols for derived units formed by multiplication are joined with a centre dot (·) or a (non-break) space, for example, "N·m" or "N m".

 

-          Symbols for derived units formed by division are joined with a solidus (/), or given as a negative exponent. For example, "metre per second" can be written "m/s", "m s-1" or as "m· s-1". Only one solidus should be used; e.g., "kg/(m·s2)" but "kg/m/s2" is ambiguous and should not be used.

 

-          Symbols are mathematical entities, not abbreviations, so do not have an appended period/full stop, i.e. “.”.

 

-          Symbols are written in upright (Roman) type (e.g. m for metres), so as to differentiate from the italic type generally used for quantities (e.g. m for mass). The consensus of international standards bodies is that this rule should be applied irrespective of the font used for any surrounding text.

 

-          Symbols for units are written in lower case (e.g., "m", "s", "mol"), except for symbols derived from the name of a person. For example, the unit of pressure is named after Blaise Pascal, and is written "Pa", even though the unit itself is written "pascal" (see below). The one exception is the litre, whose original symbol "l" is too similar to the numeral "1" or the uppercase letter "i" (for some typefaces), in some English-speaking countries. The American National Institute of Standards and Technology recommends that "L" is used instead, see NIST (2008). This usage is common in USA, Canada and Australia (but not elsewhere).

 

-          A prefix is part of the unit, and its symbol is prepended to the unit symbol without a separator (e.g. "G" in "GHz"). Compound prefixes are not allowed. All symbols of prefixes larger than 103 (kilo) are uppercase.

 

-          The 10th resolution of the General Conference on Weights and Measures (CGPM)* in 2003 declared that "the symbol for the decimal marker shall be either the point on the line or the comma on the line." In practice, the decimal point is used in English-speaking countries and most of Asia, and the comma in most continental European languages. Spaces may be used as a thousands separator (1 000 000) in contrast to commas or periods (1,000,000 or 1.000.000) in order to reduce confusion resulting from the variation between these forms in different countries. In print, the space used for this purpose is typically narrower than that between words (a ‘thin’ space). As the Nematrian website is primarily English-speaking, it generally uses the decimal point as the symbol for the decimal marker. It also generally avoids spaces within numbers, so that they are easier to copy and paste between programs.

 

-          Any line-break inside a number, inside a compound unit, or between a number and its unit should be avoided.

 

-          When writing dimensionless quantities, the terms 'ppb' (parts per billion) and 'ppt' (parts per trillion) are recognised but SI recommends avoiding these terms, since the value of billion and trillion can vary from language to language. No alternative has been suggested by the International Bureau of Weights and Measures (BIPM)*.

 

-          Names of units follow the grammatical rules associated with common nouns. In English and in French they start with a lowercase letter (e.g. newton, hertz), even when the symbol for the unit begins with a capital letter (see above). This also applies to 'degrees Celsius', since 'degree' is the unit. In German however, names of units, start with a capital letter.  Names of units are pluralised using the normal English grammar rules. For example "henries" is the plural of "henry". However, the lux, hertz, and siemens are exceptions; they are the same in singular and plural form. This rule applies only to the full names of units, not to their symbols, so two 1 kg masses have a mass of 2 kg not 2 kgs.

 

-          The official US spellings for deca, metre, and litre are deka, meter, and liter, respectively. The Nematrian unit conversion functions accept either spelling.

 

* The Metre Convention signed in 1875 established three international organisations to oversee the metric standards, i.e. (1) the General Conference on Weights and Measures (Conférence générale des poids et mesures or CGPM) which meets every four to six years with delegates from all member states, (2) the International Bureau of Weights and Measures (Bureau international des poids et mesures or BIPM), an international metrology centre at Sèvres in France; and (3) the International Committee for Weights and Measures (Comité international des poids et mesures or CIPM), an administrative committee which meets annually at the BIPM.

 


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