Take the 2-minute tour ×
TeX - LaTeX Stack Exchange is a question and answer site for users of TeX, LaTeX, ConTeXt, and related typesetting systems. It's 100% free, no registration required.

I have a document, where I want to use $\tau_{ff}$. If this is rendered it looks like a space is inserted between the two f's. $\tau_{rl}$ looks fine ...

Where does this additional space come from and how can I avoid it being inserted?

(Miktex 2.9 on windows, if this is relevant).

share|improve this question
6  
If the subscript is an abbreviation, it should be set in upright type: $\tau_{\mathrm{ff}}$ –  egreg Sep 30 '12 at 20:34
    
I have $\tau_f and $\tau_{ff}, where both are abbreviations - (ff is a special kind of f). Would you then propose to set the single f upright as well? –  Joma Sep 30 '12 at 20:52
4  
Yes; textual abbreviations in subscripts should be upright; variable names are in italics. –  egreg Sep 30 '12 at 20:59
    
@egreg Usually I think you would want to use the same upright type that the surrounding text uses. So would it be better to generally use \textnormal, rather than \mathrm? I saw this idea proposed proposed here. –  Cerran Apr 1 at 1:16
1  
@Cerran I usually propose \textnormal or \mathrm. With \mathrm you have to watch for spaces. –  egreg Apr 1 at 6:28

1 Answer 1

up vote 13 down vote accepted

Math fonts do not make ff into a ligature the way text fonts do as it may sometimes obscure the meaning of two separate identifiers juxtaposed,

enter image description here

$\tau_{ff}$ $\tau_{f\!f}$ $\tau_{\mathit{ff}}$

If the meaning of your subscript is some kind of invisible product of two f then use one of the first two, or some other negative space other than \! to taste. If on the other hand it is a multi-letter identifier with name ff then it is more appropriate to use a text font, as in the third example where the text italic font is used in math mode via \mathit and there ff produces a single ff ligature glyph.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.