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For example, in the mathematical notation of a quotient space given an equivalence relation \sim, the rendering of X/\sim is different from X/\!\!\sim. In the former, there is a gap between the slash and the \sim, whereas in the latter, there is no such gap. I couldn't find meaningful entries on Google.

Ahmed Nassar
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    \! in mathmode is a small backspace. Two of them are double that. – Steven B. Segletes Jan 25 '15 at 22:41
  • Thanks Steven. Now "backspace in math mode" gives me good results on Google. Just needed to know the right keywords. – Ahmed Nassar Jan 25 '15 at 22:45
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    It's wrong usage, anyway: if you want no space between the slash and the \sim, type X/{\sim}. The reason is that \sim is a relation symbol, while / is an ordinary one, so TeX inserts a thick space between them. With {\sim} the symbol is turned into an ordinary one, and TeX inserts no space. – egreg Jan 25 '15 at 22:48
  • @egreg: Yeah your method is more elegant. I just was annoyed by that extra space and inspected the source of a Wikipedia page (http://en.wikipedia.org/wiki/Quotient_space_(topology)) to see how they do it. They use "!!". – Ahmed Nassar Jan 25 '15 at 23:06
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    @AhmedNassar Somebody writes x/~ in order to denote the equivalence class of x. Try $X/\!\!\sim=\{x/\!\!\sim\mid x\in X\}$ and $X/{\sim}=\{x/{\sim}\mid x\in X\}$ and see the difference. Here's the picture So it's not only more elegant: it's right as opposed to wrong. ;-) – egreg Jan 25 '15 at 23:20
  • @egreg These seem to be subtle things that are great to know. I know a few but often wonder what other tricks I'm missing. It seems it would be nice if there were some kind of brief reference for such things. Is there such a reference? – A.Ellett Jan 25 '15 at 23:54
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    @A.Ellett The TeXbook or TeX by Topic. If you can read Italian, http://www.guitex.org/home/images/ArsTeXnica/AT008/Simboli%20matematici%20in%20TEX%20e%20LATEX.pdf – egreg Jan 25 '15 at 23:57
  • @egreg While I do not read Italian, there's enough context in your pdf link to figure out what they're talking about. I found the material in TeX by Topic, but it would be hard to find if you only wanted to know how to suppress the spacing. The link I find more helpful. – A.Ellett Jan 26 '15 at 00:14

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