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Although the two outputs look quite similar, what is the advantage of using $f^\prime$ instead of $f'$? By the way, here is my code:

\documentclass‎{article‎}‎
\begin{document}‎‎‎
\[‎ ‎f^‎\prime ‎(x)=y‎ ‎\]‎
\[‎ ‎f'‎ ‎(x)=y‎ ‎\]‎‎
\end{document}
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30  
There's no advantage in using $f^\prime$; it's just more awkward to type than $f'$ and the result is exactly the same. – egreg Dec 15 '12 at 17:14
up vote 62 down vote accepted

' is defined in latex.ltx as active math character:

\def\active@math@prime{^\bgroup\prim@s}
{\catcode`\'=\active \global\let'\active@math@prime}
\def\prim@s{%
  \prime\futurelet\@let@token\pr@m@s}
\def\pr@m@s{%
  \ifx'\@let@token
    \expandafter\pr@@@s
  \else
    \ifx^\@let@token
      \expandafter\expandafter\expandafter\pr@@@t
    \else
      \egroup
    \fi
  \fi}
\def\pr@@@s#1{\prim@s}
\def\pr@@@t#1#2{#2\egroup}

The active ' looks for following ' and puts them together as superscript, a''' becomes a^{\prime\prime\prime}. Thus using ' makes the input easier to write.

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Sometimes you may want to pass LaTeX code as an argument to another program. In that case the code is typically wrapped in quotes. Using quote to mean a prime will confuse the second program. For example to type TeX in graphical output of MATLAB one may use something like

str='$$F^\prime$$'
text(0,0,str,'Interpreter','latex')

to print $F'$ at location $(0,0)$. Using $F'$ in the code however becomes problematic.

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use double quotes or escapes for the String? – MaxNoe Jan 26 '15 at 10:21
1  
@MaxNoe Double quotes don't work in Matlab. String escaping is done with a double quote. So you'd have to write things like 'The symbol f'' is used to represent the first derivative of a function', which looks wrong at a first glance. All things considered, f^\prime doesn't look so bad. – Federico Poloni Feb 9 '15 at 22:18

In fact, there's a slight difference when you have two superscripts. For instance:

{\bar{e}_k}^{\prime\dag}
{\bar{e}_k}^{'\dag}

The first one fits better.

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Sure, but that's related to ' being the same as ^{\prime}, so if you have foo^{'\dag} your prime symbol is superscripted twice. {\bar{e}_k}'^{\dag} will give identical output as far as my eye can tell, but I agree your input is more elegant. Also fiddling with the my input led me to a double superscript error, which is frankly what I expected to begin with, so using foo^{\prime\dag} is definitely a nice idea here, I think – Au101 Jul 19 at 7:32
    
You should type \bar{e}_k'^{\dag} (the braces around \bar{e}_k do nothing). – egreg Jul 19 at 13:54

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