How can you change the width of a fraction bar?

I have an equation using fractions of fractions - much like

\frac{\frac{a}{b}}{\frac{c}{d}}

However, all of the fraction bars are the same width. I'd like to make the central bar wider than the others for improved readability. Is this possible? I've played around with shrinking the fond sizes of the numerator and denominator a bit, but that still doesn't improve readability.

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Can you post a minimal working example (MWE)? The output of \documentclass{article}\begin{document}$\frac{\frac{a}{b}}{\frac{c}{d}}$\end{do‌​cument} has different fraction bar lengths. Or didn't you mean the horizontal width but the line thickness? – Qrrbrbirlbel Oct 25 '12 at 14:00
When you say "width" do you mean "bar length" (horizontal width) or do you mean "bar thickness" (vertical width)? – chharvey Mar 11 '13 at 4:54

Some space can be added in the outer nominator/denominator, e.g.:

\documentclass{article}
\begin{document}
$\frac{\,\frac{a}{b}\,}{\frac{c}{d}}$
\end{document}


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Oh yes it depends what was meant by "width" (English isn't very clear here) This changes the length (or width as you view it) of the line, whereas genfrac gives control over the thickness (which would be called the width of the line in other circumstances) – David Carlisle Oct 25 '12 at 14:27

Making the line thicker using the TeX primitive:

\def\oover{\abovewithdelims...8pt}
$${a\over b} \oover {c\over d}$$
\bye


where the first . is the left delimiter, second the right, and the last argument is a dimension for the fraction bar thickness.

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You can use \genfrac from the amsmath package.

       \genfrac{left-delim}{right-delim}{thickness}{mathstyle}{numerator}{denominator}

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For orientation, the predefined values are about .4pt, .34pt and .24pt for text/display, script and scriptscript style respectively. – Qrrbrbirlbel Oct 25 '12 at 14:15
@Qrrbrbirlbel "about" or "exactly"? How can I derive the actual, precise values? – Lover of Structure Mar 4 '14 at 8:44
@LoverofStructure it is set by the fonts in use (fontdimen 8 of the font in \fam 3) see tex.stackexchange.com/questions/88991/… – David Carlisle Mar 4 '14 at 9:42
@DavidCarlisle I'm wondering why the rule thickness isn't by default scaled depending on the mathstyle. Making it easy to have a mathstyle-dependent rule thickness for \genfrac would be good. Am I missing something, or is this an oversight on part of the amsmath folks? – Lover of Structure Mar 5 '14 at 3:24
@LoverofStructure the default thickness depends on the font so is dependent on the mathstyle normally (assuming a smaller font is used in scripts) if you want to change the defaults you can set different values for fontdimen8, otherwise the intention for \genfrac was probably one-off use so normal lengths are probably easier to use than mu units – David Carlisle Mar 5 '14 at 8:26