
"One hundred and fortytwo thousand, six
hundred and twentyone" is not something that you would expect to find
in a technical paper, or indeed anywhere other than a bank draft (you
should be so lucky). However, because it's right to write 142,621 (note
the comma) doesn't mean that it's right to write "the system consists
of 1 processor, 1 memory system and 2 storage devices". You'd expect to
see "... one processor, one memory system and two storage
devices". 
So, somewhere between one and 142,621, we need
to change between words and figures. You can choose the cutoff point
for yourself, but 10 is a common value: so you write words for one to
nine, and figures for 10 up. Sometimes, you might even be justified in
writing "nought", but nought and one are special cases which we will
deal with in a moment.
 *  Naturally, there are
exceptions, such as the parts of the paper itself. No one is suggesting
you stop writing "Fig. 4", and write "Figure Four" instead.

*  Another exception is for
numbers in a technical context. No one is going to write 2.5, or any
other real number, in words, just because it happens to be between 1
and 10. Note that I just wrote "1". Even integers should be written as
figures if they are effectively constants in some mathematical
construct (here, an implied inequality), even though counting the
constructs themselves would be subject to the original convention about
figures and words. So, you might write "there are two terms, and they
sum to 2.0" If you want to emphasize that you're talking about real
numbers, it is usually better to add a decimal, even if it's a zero.

*  Nought
and one are special cases, not least because "0" and "1" look
alarmingly like a capital "O" and lowercase "l". A lot of "0"s and
"1"s in text doesn't make it easy to read. One solution is to use
"zero" (as I just did a few lines ago) and "unity", which both have
honourable mathematical careers behind them. 
 However, it is often possible to eliminate the
need to write about "one" at all. The corrected version of the example
above is more plausibly written "the system consists of a processor,
memory system and two storage devices." 
 Similarly with zero. A phrase like "there are 0
bytes of code in this routine" is easily rewritten as "this routine
contains no code". Words like "empty", "null", "void" and "absent" can
all be deployed in substituting for zero. 
