[Tex/LaTex] How to use the unit mu in an hspace

spacing

I tried combining hspace with the "math unit" mu inside a math environment, like this:

\documentclass{article}  
\begin{document}  
$\delta \hspace{1mu} f$ 
\end{document}  

But this leads to the error "Illegal unit of measure (pt inserted)." I am confused – I thought, in math mode I can (or even should?) use the unit mu?

As a workaround, I used em in math mode. With 18mu = 1em, my code is

\documentclass{article}  
\begin{document}  
$\delta \hspace{0.0555em} f$ 
\end{document}  

and compiles fine. But I still wonder – why can't I use mu?

Best Answer

In order to use mu units you need \mskip or \mkern.

If you load amsmath you have \mspace, that's preferable to \mskip in the same way \hspace is preferable to \hskip.

\documentclass{article}
\usepackage{amsmath}

\begin{document}  

$\delta \mspace{1mu} f$

\end{document}  

There is a fundamental difference between \hskip and \mskip: the latter uses the em relative to the symbol font in the current style (so it will be smaller in subscripts, for instance). In view of this there is no conversion formula, unless you want to use \mathchoice that, in this case, would be too much business for a small thing.