How to add space between variables and fraction lines

Question

In this

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\begin{document}
    \[
        \dfrac{\dfrac{a}{b}}{\dfrac{c}{d}}
    \]
    \[
        \dfrac{\dfrac{5}{12}}{\dfrac{6}{15}}
    \]
\end{document}

the fractions look very ugly, because the variables are too close to the middle fraction line. With only \frac{}{} and with numbers it looks even worse. I tried to fix it with a \raisebox but LaTeX complained (I guess it doesn't like \raiseboxes in math mode).

How can I add some space between the variables and the fraction lines?

And, while we are here, is there a way to make the middle fraction line a little longer, so that it's more obvious that it's a fraction with fractions above and below?

Answer 1

\genfrac allows you to customise the rule width, and adding a thin space allows you to control the length as well. The space above and below the rule follows the rule width.

\documentclass[12pt,a4paper]{article}

\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amssymb}

\newcommand\zfrac[2]{%
  \genfrac{}{}{1pt}{0}%
  {\mspace{1mu}#1\mspace{1mu}}{\mspace{1mu}#2\mspace{1mu}}}
\begin{document}
    \[
        \zfrac{\frac{a}{b}}{\frac{c}{d}}
    \]
    \[
        \zfrac{\frac{5}{12}}{\frac{6}{15}}
    \]
\end{document}

Answer 2

Here's a solution that uses custom macro called \ddfrac ("double dfrac"), \Bstrut ("bottom strut"), and \Tstrut ("top strut").

\documentclass[12pt,a4paper]{article}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\usepackage{mathtools} % for '\cramped' macro

\newcommand\ddfrac[2]{\frac{\displaystyle #1}{\cramped[\displaystyle]{#2}}}
\newcommand\Tstrut{\smash[b]{\strut}}
\newcommand\Bstrut{\smash[t]{\strut}}

\begin{document}
\[
\dfrac{\dfrac{5}{12}}{\dfrac{6}{15}}
\quad\text{vs.}\quad
\ddfrac{\frac{5}{12\Bstrut}}{\frac{6\Tstrut}{15}}
\]
\end{document}