[Tex/LaTex] How to properly typeset multiletter quantities, e.g. Re or Nu (Reynolds, Nusselt numbers)

best practicesmath-modetypography

Is there a canonical way to typeset such quantities in equations and inline math?

\documentclass{article}
\begin{document}
$Re$, $Nu$ are clearly not an option.
$\mathit{Re}$, $\mathit{Nu}$ appear ok to me.
\end{document}

Best Answer

When in doubt, I normally consult Knuth's publications (if I expect to find a paper that is likely to have an example of the issue at hand) and secondly the relevant Journal author guidelines (in that order).

Sometimes the latter will have some badly made templates, but most probably would have strict and stroppy editors. One caveat is that if you aiming at eventually publishing the document the editor is likely to have the final say and I would suggest it is unwise to get into an argument about typography with the editor.

I guess that the OP is dealing with Fluid Mechanics. The Journal of Fluid Mechanics has instructions as to how to typeset these dimensionless numbers, including a template and instructions.

Since these are essentially numbers I would recommend the approach that they are typeset in math italic font. It also looks better if you are describing any of these numbers inline.

The above cited journal has them defined as:

\newcommand\Rey{\mbox{\textit{Re}}}  % Reynolds number
\newcommand\Pran{\mbox{\textit{Pr}}} % Prandtl number, cf TeX's \Pr product
\newcommand\Pen{\mbox{\textit{Pe}}}  % Peclet number

which, I agree with egreg is not very wise. However, since they provide the .cls is not the end of the world and the template works.

Personally I recommend you use the \DeclareMathOperator from amsmath (if you worry about spacing-personally I wouldn't). Also my own preference is to use a notation such as, N_{\mathrm{Re} rather than a double symbol such as Re.

As a final word, I think consistency is the key and that you should use the notation that is most familiar with your readers.