# Equations: Why does formula symbol index notation “ff” cause such big letter spacing?

The formula symbol index notation ff is causing some ugly letter spacing in comparison to other index notation letters.

Minimum Working Example (MWE):

\documentclass{article}

\begin{document}

$$t=\frac{h\varepsilon_{eff}}{w}$$

$$t=\frac{h\varepsilon_{ett}}{w}$$

\end{document}


Screenshot of the result:

Description of the issue:

As you can see, the letter spacing distance between those two f's is very huge in comparison to the t's of the second equation. Therefore the second equation appears well balanced and a bit more nicely than the first one.

Do you all write eff the way I'm doing, or is there some special trick to reduce this ugly letter distance?

Update (2019/01/19): The user marmot has posted an approach of writing the index notation in text mode:

t=\frac{h\varepsilon_\mathrm{eff}}{w}


I am not sure if it is typographically allowed to write index notations in text mode? I thought in equations everything has to be italic, no?

• You probably want t=\frac{h\varepsilon_\mathrm{eff}}{w}. – marmot Jan 19 at 21:48
• OK, I thought eff would stand for effective. If these are indices, you are of course right. – marmot Jan 19 at 21:52
• @Dave: I can hardly believe that the “eff” in your formula is meant to denote the product of the three quantities “e’ and “f squared”. Contrary to what you seem to believe, indices that denote words (like “effective”, or “efficacious”) must be written, in formulas, in upright (Roman) font. – GuM Jan 19 at 21:56
• Yes, of course. Texts should always be typeset upright, only symbols are italic AFAIK. (I personally would do t=\frac{h\,\varepsilon_\mathrm{eff}}{w} but there are users who do not like the \,). – marmot Jan 19 at 21:57
• No, I repeat, it is common practice to write that particular type of pedices upright, and I think ISO regulations actually mandate so (I cannot swear for this, I’m not particularly fond of ISO regulations, they also dictate that the “d” in \int f(x)\,dx should be upright, something I’ll never yield to unless under threat of death). I also wanted to remark about using \mathit, but @DavidCarlisle has already taken care of this! – GuM Jan 19 at 22:11

You can use italic but use \mathit{eff} never use the default math italic for multi-letter identifiers it is designed to make adjacent letters look like a product of variables and not a word. Alternatively you can use \mathrm{eff} if you want upright. Both of those fonts are designed for words.
• @Dave: If the subscript denotes a variable, it should be in math italic (e.g., m_{i} for the i-th mass), but if the subscript is an abbreviation for a word, best practice (and, I repeat, probably also ISO directive) is to set it upright (e.g., m_{\mathrm{E}} for the mass of the Earth). – GuM Jan 19 at 22:22