%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-1-17 on page I-270 in "The placement of the item label or item text".
%%
%% Copyright (C) 2022 Frank Mittelbach
%%
%% It may be distributed and/or modified under the conditions
%% of the LaTeX Project Public License, either version 1.3c
%% of this license or (at your option) any later version.
%%
%% See https://www.latex-project.org/lppl.txt for details.
%%

\documentclass{tlc3exa}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{159.0pt}


  \usepackage{microtype} % to improve h&j


%StartShownPreambleCommands
\usepackage{calc,enumitem}
%StopShownPreambleCommands

\begin{document}
\noindent A sample text showing the outer margins.
\begin{description}
\item[X] Label shorter than the available space so
  the text moves to the left.
\item[Wide label] Wider than the space available so
  the text moves to the right.      \end{description}

\setlist[description]{labelwidth=\leftmargin-\labelsep}
\begin{description}
\item[X] Now no text is moved.
\item[Wide label] Wider than the space available so
  the text again moves to the right.\end{description}
\end{document}