%%
%% The LaTeX Companion, 3ed
%%
%% Example 3-2-6 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{underscore}
%StopShownPreambleCommands

\begin{document}
\fbox{\parbox{77pt}{
  A compound\textunderscore fracture \\ % bad
  A compound\_fracture               \\ % good (with package)
  A bad compound\_fracture           \\ % good? see below
  A really bad compound\_fracture       % ok here
}}

\end{document}