Usage Conventions in SI

There are various rules laid down for the use of SI and its units as well as some observations to be made that will help in its correct use.

Any unit may take only a single prefix. For example 'millimillimetre' is incorrect and should be written as 'micrometer'.

Prefixes which make a unit bigger are written in capital letters (M G T etc.), but when they make a unit smaller then lower case (m n p etc.) is used. The one exception to this is kilo [k] to avoid any possible confusion with kelvin [K].

A unit which is named after a person is written all in lower case (newton, volt, pascal etc.) when named in full, but using a capital letter (N V P etc.) when abbreviated. An exception to this rule is the litre which, if written as a lower case 'l' could be mistaken for a '1' (one) and so a capital 'L' is allowed as an alternative.

 

Units written in abbreviated form are never pluralised. So 'm' could always be either 'meter' or 'metres'. 'ms' would represent 'meter second'.

 

To make numbers easier to read they may be divided into groups of 3 separated by spaces (or half-spaces) but NOT commas.

 

The SI preferred way of showing a decimal fraction is to use a comma (123,456) to separate the whole number from its fractional part. The practice of using a point, as is common in English-speaking countries, is acceptable providing only that the point is placed ON the line of the bottom edge of the numbers (123.456).

 

It will be noted that many units are eponymous, that is they are named after persons. This is always someone who was prominent in the early work done within the field in which the unit is used.