%% 
%%  Der Mathematiksatz mit LaTeX, 1. Auflage 2009
%% 
%%  Example 8-3-12 on page 173.
%% 
%%  Copyright (C) 2009 Herbert Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

\documentclass[]{ttctminimal}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{170.59192pt}
\AtBeginDocument{\setlength\parindent{0pt}}
\StartShownPreambleCommands
\def\yPh{\vphantom{y}}
\let\ul\underline
\StopShownPreambleCommands
\begin{document}
$\underline{x}+\underline{y}=\underline{z}$\\[8pt]
$ \ul{x\yPh} + \ul{y} = \ul{z\yPh} $\\[8pt]
$ \ul{x_1} + \ul{y_2} = \ul{z_3} $
\end{document}