There are various rules laid down for the use of the SI and its units as well
as some observations to be made that will help in its correct use.
- Any unit may take only ONE prefix. For example 'millimillimeter' is
incorrect and should be written as 'micrometer'.
- Most 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. Exceptions to this are the kilo [k] to avoid any possible confusion
with kelvin [K]; hecto [h]; and deca [da] or [dk]
- 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. Such a unit is written all
in lower case (Newton, volt, Pascal etc.) when named in full, but starting
with a capital letter (N V Pa etc.) when abbreviated. An exception to this
rule is the liter 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. It is
intended that a single letter will be decided upon some time in the future
when it becomes clear which letter is being favored most in use.
- Units written in abbreviated form are NEVER pluralized. So 'm' could
always be either 'metre' or 'metres'. 'ms' would represent 'millisecond'.
- An abbreviation (such as J N g Pa etc.) is NEVER followed by a full-stop
unless it is the end of a sentence.
- 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) and NOT in the middle.