Should thin spaces be used between numerals and units

Question

After starting to use the siunitx package for typesetting units (and the numerals before the units) in LaTeX, I noticed that it typesets a single space between a numeral and a unit (a space that is not as wide as a normal space). I have tried unsuccessfully to find a proper reference that calls for this. Wikipedia mentions it on Space (punctuation), but the external reference given on the Wikipedia page, which is in this online .pdf brochure, does not say anything about thin spaces. It only mentions that there should be a space between numerals and units, but says nothing about the size of that space (in which case I think it is reasonable to assume that a normal space is referred to). I have added a question about this in the talk section for the Space article.

What is the proper size of spaces between numerals and units? It would be interesting to know the source for the usage of thin spaces in siunitx, and if that is the way it "really should be".

Answer 1

As I understand the official SI manual (which siunitix is supposed to follow), there should be a space, not a thin space, between the number and the unit:

The numerical value always precedes the unit, and a space is always used to separate the unit from the number. (section 5.3.3)

I also find that this seems to be the more common procedure in typed texts (but that might again be because most people don't know abut the thin space).

Just to be clear, the SI manual is aware of the thin space, cf. the following section 5.3.4:

Following the 9th CGPM (1948, Resolution 7) and the 22nd CGPM (2003, Resolution 10), for numbers with many digits the digits may be divided into groups of three by a thin space, in order to facilitate reading.

And precisely for this reason, the siunitix package has since this question was asked changed its setting from using a thin space to using a normal space, see this update.

Answer 2

You can use either of U+202F NARROW NO-BREAK SPACE or just plain U+00A0 NO-BREAK SPACE, but you certainly should not let it risk being line-broken. I’m not so sure that the thinness matters half so much as the no-break property. You do not want to let the figures get orphaned without their units.

Answer 3

This is an excellent English language question. On the theory that however D. Knuth did it is correct, turn to page 113 of the TeXbook, which contains the following source.

  The notation |b=|^|*| and
  |c=*| on the final line means that $b$ and~$c$ are infinite; the total
  height of $553\pt$ cannot be reduced to $528\pt$ by shrinking the
  available glue. Therefore the page is ejected at the best previous place,
  which turns out to be a pretty good break: |b=5| and |p=-100| yield a
  net cost of $-95$.

You'll see that he writes "553 pt" in math mode with no space between the number and the macro \pt. \pt is defined as follows:

\def\pt{\,{\rm pt}} % units of points, in math formulas

\, is equivalent to \mskip \thinmuskip, which means to insert math mode kerning equal to \thinmuskip, which is defined as 3 mu (= 1/6 em, no stretching or shrinking permitted). The macro \thinspace (used outside of math mode) inserts 1/6 em of kerning as well.

Line breaks are generally avoided in in-line math formulas, so this means that the space will be non-breaking.