How to build two lines aligned maps

I would like to write a map between two objects, with a map below which specify where any single object is sent. To be more precise. I want to write $A \rightarrow B$, and right below it $\alpha \mapsto \beta$. I tried with the command $\underset$ but the maps below gets too smaller, and it's not aligned to the map above. What is the right command?

I apologize if this question has been already asked. To me it looks elementary, nevertheless I couldn't find it nor here nor googling around. Thank you very much for your time!

• Welcome to TeX.SX. A tip: If you indent lines by 4 spaces, then they're marked as a code sample. You can also highlight the code and click the "code" button ({}). Jan 23 '13 at 16:54
• @ClaudioFiandrino, Thank you for your suggestion, I'll stick to it for future questions! Jan 23 '13 at 16:55

Something like this or variants like aligned if you want it to be part of a larger expression.

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
A&\rightarrow B\\
a&\mapsto b
\end{align*}

\end{document}

• rats, ya beat me. but you might want to mention commutative diagram packages like xypic or tikz for more complicated diagrams. Jan 23 '13 at 17:02
• @barbarabeeton and not forgetting amscd ? Jan 23 '13 at 17:05
• amscd is intentionally limited. it only handles strictly horizontal or vertical arrows, so if anything slanted is needed, forget it. better to start with something more functional. Jan 23 '13 at 17:12

amsmath and mathtools offers extensible arrows. (If I get the question correctly)

\documentclass{article}
\usepackage{mathtools}

\begin{document}
$A \xrightarrow[\alpha \mapsto \beta]{} B$
\end{document}