For sure we need to try to follow SI as close as possible, but sometimes
keyboard limitations just force us to use some close representation, as
for example "u" for micro, even that I can type ALT+0181 ==> µ <=== and
it appears at my email text as the micro symbol, probably you can't see
it between the arrows, right?... then we just use "u".

John Payson wrote:
> 
> |Note : Alternative definitions of the SI prefixes and their symbols are
> |not permitted. For example, it is unacceptable to use kilo (k) to
> |represent 2^10 = 1024, mega (M) to represent 2^20 = 1 048 576, or giga
> |(G) to represent 2^30 = 1 073 741 824.
> 
> Table 5. SI prefixes
> 
> Factor      Prefix        Symbol
> ------  ----------------- ------
> 10^-6  = (10^3)^-2  micro  ?
> 
> Is there any official standard which permits usefulness of a lowercase
> "u" in character sets where the "mu" is not available?  Certainly, there
> is no linguistic relationship between the characters, but the "u" is the
> one that looks most like a "mu" and I've seen it used for that purpose
> an awful lot...