%%
%% The LaTeX Companion, 3ed
%%
%% Example 4-2-35 on page I-318 in "Defining your own variants".
%%
%% 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{183.0pt}

\addtolength\textwidth{-30pt}

%StartShownPreambleCommands
\usepackage{fancyvrb}
\CustomVerbatimEnvironment{myverbatim}{Verbatim}
        {numbers=left,frame=lines,framerule=2pt}
%StopShownPreambleCommands

\begin{document}
\begin{myverbatim}
The normal case with thick
rules and numbers on the left.
\end{myverbatim}
\begin{myverbatim}[numbers=none,framerule=.6pt]
The exception without numbers
and thinner rules.
\end{myverbatim}
\RecustomVerbatimEnvironment{myverbatim}{Verbatim}
         {numbers=left,frame=none,showspaces=true}
\begin{myverbatim}
And from here on the environment
behaves differently again.
\end{myverbatim}
\end{document}