How to write two subscripts on top of each other in math mode?

Question

I want to write something like

Q_{a\in A,b\in B}

but with $b\in B$ being below $a \in A$. What is the way to do so?

Answer 1

Use \substack:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\[ Q_{\substack{a\in A\\b\in B}} \]%

 \end{document} 

Answer 2

Use the \genfrac command from the amsmath package.

\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
Q_{\genfrac{}{}{0pt}{}{a\in A}{b\in B}} 
\end{equation*}
\end{document}

Answer 3

Using align enviroment:

\documentclass[a4paper,12pt]{article}
\usepackage{amsmath,amssymb}
\begin{document}
$Q_{\begin{aligned}
\scriptscriptstyle a & \scriptscriptstyle\in A\[-1em]
\scriptscriptstyle b  & \scriptscriptstyle\in B
\end{aligned}}$
\end{document}