# 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}.
– user121799
Jan 19, 2019 at 21:48
• OK, I thought eff would stand for effective. If these are indices, you are of course right.
– user121799
Jan 19, 2019 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, 2019 at 21:56
• 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 pf variables and not a word. Jan 19, 2019 at 22:05
• 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, 2019 at 22:11

You can make this italic—but use \mathit{eff}. Never use the default math italic for multi-letter identifiers. That is designed to make adjacent letters look like a product of variables, not a word. Alternatively, you can use \mathrm{eff} if you want upright. Both of those fonts are designed for words.

• Thank you very much! So this is valid for multi-letter identifiers, but for single-letter identifiers I am still allowed to keep up the math mode?
– Dave
Jan 19, 2019 at 22:15
• @Dave yes sure you should use the math italic font for single letters. All variables should be called x all functions should be called f and math fonts are optimised for single letter identifiers, it's the way it is:-) Jan 19, 2019 at 22:18
• @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, 2019 at 22:22
• @GuM when I did math for a living, I never referred to anything that related to a real word (or the real world:-) Jan 19, 2019 at 22:24
• @GuM there are no ISO standards for typesetting mathematics in general, unfortunately ISO 80000 is sometimes interpreted that way, but that's just notational conventions for one particular subject area of engineering and has no relevance to mathematical typesetting in general Jan 19, 2019 at 22:26

Another approach would be to use negative thin space \!. With this you can reduce the space between the letters without invoking the mathit or mathrm. You can adjust the spaces as needed.

\documentclass{article}
$$t=\frac{h \, \varepsilon_{e\!f\!\!f}}{w}$$
$$t=\frac{h\varepsilon_{ett}}{w}$$