Spacing between (multicharacter) math symbols that are in upright font

Question

In environmental science (and probably also other disciplines), it is a fairly common practice to use multi-character symbols for variables (often these may be descriptive acronyms of what the variable is). For example, carbon use efficiency might be given the symbol CUE.

Multi-character symbols are typically typeset using an upright font rather than italics. For example, $\mathrm{FOO}$. Unfortunately this (possibly frowned upon by purists) practice plays havoc with the spacing in math mode. This is particularly evident when two upright symbols are multiplied together.

An example:

\documentclass[12pt]{article}
\begin{document}
This has no spacing: $x = \mathrm{FOO} \mathrm{BAR}$

This has too much spacing: $x = \mathrm{FOO} \quad \mathrm{BAR}$
\end{document}

produces

What is the best practice for typesetting multi-character symbols in math to avoid this issue?

Note that using math italics does not help (even if it were corresct to do so) as this introduces space between each character as though they were each separate variables.

Answer 1

Depending on the real use case a math class such as \mathop used here may result in correct spacing being applied automatically.

\documentclass[12pt]{article}
\usepackage{amsmath}
\DeclareMathOperator\FOO{FOO}
\DeclareMathOperator\BAR{BAR}
\begin{document}
This has no spacing: $x = \mathrm{FOO} \mathrm{BAR}$

This some spacing: $x = \mathrm{FOO} \,\mathrm{BAR}$


This automatic spacing: $x = \FOO\BAR$
\end{document}