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Are there any conventions for using two-letter subscripts in equations?

I need to use fr for friction, and the normal math italic in TeX ($\omega_{fr}$) doesn't look very good, it appears like f and r are two unrelated concepts hanging out together rather than a unified thing.

\documentclass[border=1mm]{standalone}
\begin{document}
$\omega_{fr}$ vs.
$\omega_{\mathrm{fr}}$ vs.
$\omega_{f\!r}$ vs.
$\omega_{\!f\!r}$
\end{document}

enter image description here

Note: this question is not how to accomplish typesetting in TeX (which I can ask on tex.stackexchange.com) but how it should be typeset, and where I might find this kind of information from a reputable source.

I found the AMS Style Guide online and it states "Stet italic superscript and subscript functions if used consistently." --- that is using $x_{max}$ ($x_{max}$) is OK if done consistently rather than $x_{\max}$ ($x_{\max}$) --- as well as in Appendix A, "These functions and operators are set in roman type in both roman and italic text; however, you may stet superscript and subscript italic functions and operators." (Which I would interpret as, "Authors should put well-known functions and operators in roman type, but if you really want to leave them in regular italics, we won't complain.") But there's no guidance about other multicharacter subscripts.

Jason S
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    I like $\omega_{\mathrm{fr}}$, the second option you listed using mathrm. – littleO Feb 08 '23 at 18:27
  • (side note: please do not close as duplicate of https://math.stackexchange.com/questions/2994925/typographic-conventions-for-subscripts which is a very broad question and has no answers) – Jason S Feb 08 '23 at 18:28
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    I also much prefer the mathrm version, though I've got no consistent standard to base my choice on. – JonathanZ Feb 08 '23 at 18:34
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    The first one effectively indicates a product $f \cdot r$, which is why the spacing is wider than you'd like. If you go with the italic version, then rather than playing with the spacing, how about using \textit? Here is _{fr} vs _\textit{fr}: $$\huge \omega_{fr} \quad \omega_\textit{fr}$$ Similarly, if you were to use $x_\textit{max}$, then it should be x_\textit{max}, because x_{max} suggests a product $x_{m\cdot a\cdot x}$. There's a subtle difference between the two: $$\huge x_{max} \quad x_\textit{max}$$ – Théophile Feb 08 '23 at 19:08
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    Are you aware there's a difference between $\omega_{friction}$ ($\omega_{friction}$) and $\omega_{\mathit{friction}}$ ($\omega_{\mathit{friction}}$)? I use the whole word "friction" so it becomes more apparent. I'm not sure if this is too "typesetting" advice for you, so I'm just leaving a comment. The difference becomes more apparent when being rendered by proper $\LaTeX$. As I understand it, \mathit is what one usually uses for a multiple-letter identifier. $friction$ gets typeset as if it is the product of 8 one-letter variables, rather than as if it is a word in its own right. – Izaak van Dongen Feb 08 '23 at 19:10
  • @Théophile, I think \mathit is slightly preferred to \textit, essentially for the reasons discussed here (\textit will for instance use the "main" text font, which might be a bit weird if you're using a different font for maths.) – Izaak van Dongen Feb 08 '23 at 19:17
  • @IzaakvanDongen Ah, good to know, thanks. – Théophile Feb 08 '23 at 19:27
  • I was under the impression \mathit would use mathematical spacing but the example clearly has text spacing. Interesting... – Zhen Lin Feb 08 '23 at 23:29

2 Answers2

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Looks like there are a couple of style guidelines in other related fields (physics, electrical engineering) that do make clear recommendations:

  • NIST SI unit rules

    Superscripts and subscripts are in italic type if they represent variables, quantities, or running numbers. They are in roman type if they are descriptive.

    (although IMHO the example given is really lousy, $c_p$ for specific heat capacity at constant pressure, "p" = pressure, but $m_{\mathrm{p}}$ for mass of a proton, "p" = proton)

  • Physical Review Style and Notation Guide

    Abbreviations in subscripts and superscripts fall into two categories: (1) single-letter and (2) multiletter abbreviations. Most single-letter abbreviations are conventionally printed in the italic font, i.e., $E_C$ where $C$ stands for Coulomb. Multiletter abbreviations are conventionally printed in the roman font whether they represent one or more words, i.e., $E_{\mathrm{lab}}$, where lab stands for laboratory (truncated word—lowercase) and $E_{\mathrm{HF}}$, where HF stands for Hartree and Fock (acronym), two proper names. Please note that you should always capitalize abbreviations that represent proper names.

  • IEEE Editorial Style Manual

    Displayed Equations
    Certain types of material in displayed equations are automatically italic. Some simple general rules apply. All variables are italic. (e.g., $x$, $y$, $n$). Function names and abbreviations are Roman (sin, cos, sinc, sinh), as are units or unit abbreviations (e.g., deg, Hz,) complete words (e.g., in, out), and abbreviations of words (e.g., max, min), or acronyms (e.g., SNR). Single letter superscripts and subscripts may be italic even if they are abbreviations, unless this leads to inconsistency between italic and roman characters for similar types of subscripts.

Jason S
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0

As you point it out, the main problem is the ambiguity of the index $\mathrm{fr}$, that may appear like two unrelated symbols $f$ and $r$. Looking at the Wikipedia entry for Energy of friction, I found a formula using $\mathbf{F}_{\mathrm{fric}}$ for the friction force. All of a sudden, the ambiguity of $\mathrm{fr}$ disappears when you use $\mathrm{fric}$ instead of $\mathrm{fr}$, and whether to use italics, slanted or roman becomes a lesser issue. I would follow @littleO and use $\mathrm{fric}$, but in order to ensure a consistent use, I would define a macro (that you could reuse in other articles)

\newcommand{\fric}{\mathrm{fric}}

and then use

$\omega_\fric$

Note that the other example you mention is already unambiguous. Everybody will understand if you write, for instance

The temperature $T$ ranges between $T_{\mathrm{min}}$ and $T_{\mathrm{max}}$.

In my opinion, avoiding ambiguities should be your first priority (and not only in mathematical texts...), even at the cost of a slightly longer formula.

J.-E. Pin
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