%%
%% The LaTeX Companion, 3ed
%%
%% Example 3-2-7 on page I-152 in "underscore --- Making that character more usable".
%%
%% 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{111.0pt}

%StartShownPreambleCommands
\usepackage[nohyphen]{underscore}
%StopShownPreambleCommands

\begin{document}
\fbox{\parbox{77pt}{
  A compound\_fracture    \\
  A bad compound\_fracture     % no hyphen here this time
}}
\end{document}