# commath, \dfrac and \tfrac

I'm using the commath package for its nice \od and \pd functions, but I find it uses \tfrac internally. It seems to have an 'if-then' clause to sometimes use \dfrac but it never does so in my tests:

test 1: \od{p}{x}\\
test 2: $\od{p}{x}$
test 3: $$\od{p}{x}$$


Or in align environments either. I don't see any options to control the package behavior. Should I just re-define \od and \pd in my preamble to behave the way I want?

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Where do you want it to use \dfrac? It does so in display style $...$. –  Werner Jan 23 '12 at 5:10
Most of my equations are in align environments. I use this by default. Should I be using something else? –  mankoff Jan 23 '12 at 6:08
So am I right in assuming you are interested in having more control over using \dfrac and \tfrac, and not necessarily an explanation of why this is the case. Are you interested in a redefinition that uses (say) starred \od*/\pd* for \dfrac usage, and \od/\pd for the \tfrac usage? –  Werner Jan 23 '12 at 6:34
I guess I'm looking for both... The normal size fractions look better to me, so I'd like to use them with the simpler forms provided by commath. I can redefine the commands myself. I'm wondering if there is some way to do this for commath via an option that I've missed. At the same time, if I'm breaking a standard, I'd be happy for an explanation why I should be happy with it the way it is, and that I should change all my existing \frac to \dfrac or \tfrac instead... –  mankoff Jan 23 '12 at 6:45
Similarly, why does commath use one font for \[ environments, and another for \begin{align}. Aren't they similar enviroments? I understand the difference for $ and $$ compared to \[. – mankoff Jan 23 '12 at 6:46 ## 1 Answer commath provides \textstyle and \displaystyle versions of its \od and \pd (and \md) commands: • \textsyle override: \tod and \tpd • \displaystyle override: \dod and \dpd \documentclass{article} \usepackage{commath}% http://ctan.org/pkg/commath \begin{document} \verb|Normal:| \par test 1: \od{p}{x} \par test 2: \od{p}{x} \par test 3: \(\od{p}{x}$$ \par \bigskip \verb|\textstyle-version:| \par test 1: \tod{p}{x} \par test 2:$\tod{p}{x}$\par test 3: $$\tod{p}{x}$$ \par \bigskip \verb|\displaystyle-version:| \par test 1: \dod{p}{x} \par test 2:$\dod{p}{x}\$ \par
test 3: $$\dod{p}{x}$$ \par
\end{document}


It would be possible to define your own commands doing the same with a slightly different interface (using xparse, say) that could allow you some flexibility.

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Can you explain the difference between the "Normal" version and the "textstyle" (t) version? It took some serious squinting before I even noticed a difference. –  Tyson Williams Jun 26 '13 at 21:41
@TysonWilliams: There is no difference. Perhaps what you see is an artefact from my viewer during the screen grab. \tfrac is a forced \textstyle\frac, while \dfrac is a forced \displaystyle\frac. The "normal" \frac is either \textstyle or \displaystyle, depending on the style it is used in. The same holds for the *-\od family above. –  Werner Jun 26 '13 at 21:44