Q:

Sub tilde in LaTeX

In LaTeX, \tilde{x} puts a ~ on top of the x.
How can I put a ~ below the x?

A:

Use the command \utilde from the undertilde package.
Added by the crowd: This package is not part of TeX Live, so you may have to install it manually. This having been done, using the package is simple:
\documentclass{article}
\usepackage{undertilde}
\begin{document}
$\utilde{x}$
\end{document}

This produces the following output.

A:

Or just copy the 4 lines of code:
\def\undertilde#1{\mathord{\vtop{\ialign{##\crcr
$\hfil\displaystyle{#1}\hfil$\crcr\noalign{\kern1.5pt\nointerlineskip}
$\hfil\tilde{}\hfil$\crcr\noalign{\kern1.5pt}}}}}

from 
undertilde.sty 

and use the command 
\undertilde{x}

