# Is there an “underscript” command?

Is there a way to do underscripts? By "underscript" I mean a small font underneath the text object like limits or sums have. I don't want it to behave like a math operator though, where it is typeset as a subscript or underscript depending on the environment (like sums). I want there to always be an underscript in math environments with the possibility of adding a subscript in addition to the underscript. Is there anything like an \underscript{X}{a} command?

Is there even something like that that will only work in mathmode? I haven't been able to find it.

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I think $\underset{a}{X}$ is what you are looking for? There is also $\stackrel{A}{x}$ but that produces smaller text. –  Peter Grill Nov 16 '11 at 18:06
Yes, \underset{}{} sounds like the command you want but you'll need to put \usepackage{amsmath} in the preamble. –  DJP Nov 17 '11 at 0:15

I would recommend using \underaccent from the accents package. For comparison purposes I have included the output \underset and \stackrel:

\documentclass{article}
\usepackage{amsmath}
\usepackage{accents}
\usepackage{xcolor}

\begin{document}
$\quad\color{blue} \text{underset: } \underset{x}{A}$
$\quad\color{black}\text{stackrel: } \stackrel{A}{x}$
$\quad\color{red} \text{underaccent: } \underaccent{x}{A}$
\end{document}

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While the stackengine package is not really designed for math mode, it can work in math mode, if the arguments are set in math delimiters. It gives the flexibility to control the vertical gaps (as either an interelement gap or as a baseline shift), the left-center-right alignment of the stack, whether the stack is built up or down from the anchor of the stack, whether it prints the output (or merely saves it for later use), and whether the width of the stack is defined by the widest element of the stack or merely by the width of the anchor element of the stack.

\documentclass{article}
\usepackage{stackengine}
\begin{document}
\stackengine{4pt}{$A$}{$x$}{U}{c}{F}{T}{S}
\stackengine{2pt}{$A$}{$\scriptstyle x$}{U}{c}{F}{T}{S}
\stackengine{2pt}{$A$}{$\scriptscriptstyle x$}{U}{c}{F}{T}{S}
\end{document}


The \stackengine command is the basic command of the package with all the options to tweak. Often, a shortcut command can be used instead, for example \stackunder[2pt]{$A$}{$\scriptscriptstyle x$} will produce the last item from above.

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