How to place symbols on top of each other (not \overset, \underset)?

Question

I tried to set two symbols on top of each other via:

\begin{align*}
  \Rightarrow\atop\nLeftarrow
\end{align*}

How can I reduce the space between the symbols? \overset etc. is what I normally use, but that reduces the size of one of the symbols and I would like them to be of the same size.

Answer 1

Use the \genfrac command from the amsmath package.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\genfrac{}{}{0pt}{0}{\Rightarrow}{\Leftarrow}
\]
\end{document}

Another possibility is to use an array environment. In this case an optional argument to the \\ macro allows you to manually adjust the vertical spacing.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[
\begin{array}{c}
\displaystyle\Rightarrow \\[-5pt]
\displaystyle\Leftarrow
\end{array}
\]
\end{document}