What is the macro used to define \underline{}?

Question

How is the \underline command defined? And how can one make a custom underline to be used just as a horizontal line but which respects a specified height or the height of text, math input, etc and of a specific width?

Here is my first attempt:

\newcommand{\ndfracunderline}[1]{\ensuremath{\underline{\phantom{\dfrac{2}{3\hspace*{#1}}}}}}

This works just fine but the following does not:

\newsavebox\underlinebox

\newcommand\cunderline[3][black]{%
\newlength{\cunderlineheight}%
\settoheight{\cunderlineheight}{#3}%
{\sbox\underlinebox{\cunderlineheight\hspace*{#2}}%
  \rlap{\usebox\underlinebox}\color{#1}\rule[-1.2\dp\underlinebox]{\wd\underlinebox}{0.5pt}}}

Answer 1

I don't know whether this might be what you're after:

\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\usepackage{xcolor}% http://ctan.org/pkg/xcolor
\makeatletter
\newcommand{\newunderline}[3][black]{% \newunderline[<colour>]{<width>}{<stuff>}
  {\def\@tempa{!}%
   \def\@tempb{#2}%
   \ifx\@tempa\@tempb\relax
     \def\@tempa{\phantom}\let\maybe@hspace\@gobble%
   \else%
     \def\@tempa{\vphantom}\let\maybe@hspace\hspace%
   \fi%
   \color{#1}\underline{\@tempa{#3}\maybe@hspace{#2}}}}
\makeatother

\begin{document}

$x$: \newunderline{1ex}{$x$} \par
$\frac{x}{y}$: \newunderline[red]{2em}{$\frac{x}{y}$} \par
$\dfrac{\mathrm{d}y}{\mathrm{d}x}$: \newunderline[blue!80!white]{!}{$\dfrac{\mathrm{d}y}{\mathrm{d}x}$}

\end{document}​​​​​​​​​​​​​​​​​​​​​​​​​

\newunderline[<colour>]{<width>}{<stuff>} takes 3 arguments. The width is set to the natural width of <stuff> if its supplied as !. According to your request, it respects the depth of the argument, the mode (text or math) and can be of specified width (or natural).