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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).

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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
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 '14 at 1:16
@Cerran I usually propose \textnormal or \mathrm. With \mathrm you have to watch for spaces. – egreg Apr 1 '14 at 6:28
up vote 14 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.

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