%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-3-9 on page I-339 in "lineno --- Numbering lines of 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{141.0pt}

  \setlength\marginparsep{20pt}
  \addtolength\textwidth{-30pt}

  \usepackage{microtype} % improve h&j
  \tolerance=4000

%StartShownPreambleCommands
\usepackage{lipsum,lineno} \linenumbers
\newenvironment{numarrows}
  {\par\renewcommand\makeLineNumber
          {\llap{\LineNumber$\to$ }}}{\par}
\newenvironment{arrows}
  {\par\renewcommand\makeLineNumber
          {\rlap{\hspace{\textwidth} $\gets$}}}{\par}

%StopShownPreambleCommands

\begin{document}
\begin{numarrows} \lipsum[1][1]  \end{numarrows}
\begin{arrows}    \lipsum[2][1]  \end{arrows}
\textbf{This paragraph has normal numbering.}
\begin{numarrows} \lipsum[3][1]  \end{numarrows}
\end{document}