Why not catcode11 (letter) but catcode12 (other) is used for verbatim?

Have a look at how the verbatim environment is implemented in the commented source code of LaTeX 2ε which can be found in source2e.pdf (https://ctan.org/pkg/source2e).The verbatim-environment is in File y: ltmiscen.dtx, section 53.3 Verbatim.

There you can see that the macro \verbatim, which is called by\begin{verbatim},

  • first does call the command \@verbatim which in turn does a lot of preparation like performing required catcode-changes (by means of \let\do=\@makeother and \dospecials), adding vertical space and the like,
  • then calls \frenchspacing for having all spaces in equal width,
  • then does call \@vobeyspaces for turning spaces into something where no line-breaks occur,
  • then does call \@xverbatim.

\@xverbatim in turn does "catch" the text that is to be typeset verbatim in terms of a delimited argument, whereby the delimiter consists of a sequence of character tokens \, e, n, d, {, v, e, r, b, a, t, i, m, }, whereby the catcode-régime at the time of tokenizing the delimiter within the definition-text of \@xverbatim is set as follows:

\begingroup \catcode ‘|=0 \catcode ‘[= 1
\catcode‘]=2 \catcode ‘\{=12 \catcode ‘\}=12
\catcode‘\\=12 |gdef|@xverbatim#1\end{verbatim}[#1|end[verbatim]]
|endgroup

This means \@xverbatim "expects" the character-tokens \ and { and } from the delimiting phrase \end{verbatim} to have catcode 12 (other).

\@xverbatim will spit out its argument trailed by the sequence |end[verbatim] where |end is the control word \end and [ and ] are of category code 1 respective 2 and thus serve as opening braces/closing braces so that the entire thing is the \end-command for the environment.

For fun you can say 

\def\@makeother#1{\catcode`#1=11\relax}

and

\begingroup
\catcode`|=0  \catcode`[=1   \catcode`]=2
\catcode`{=11 \catcode`\}=11 \catcode`\\=11 
|@firstofone[%
  |endgroup
  |def|@xverbatim#1\end{verbatim}[#1|end[verbatim]]%
]%

and see that now the verbatim-environment works with catcode-11-characters also:

\documentclass{article}
\begin{document}
\begingroup
\makeatletter
\def\@makeother#1{\catcode`#1=11\relax}%
\begingroup
\catcode`|=0  \catcode`[=1   \catcode`]=2
\catcode`{=11 \catcode`\}=11 \catcode`\\=11 
|@firstofone[%
  |endgroup
  |def|@xverbatim#1\end{verbatim}[#1|end[verbatim]]%
]%
\makeatother
\begin{verbatim}
\{}[]~^_-+#'blabla
\{}[]~^_-+#'blabla
\{}[]~^_-+#'blabla
\end{verbatim}
\endgroup
\end{document}

enter image description here


Well, characters from A to Z and from a to z do retain their \catcode (11) in the verbatim environment. I see no particular contraindications to setting also the \catcodes of other symbols to 11, except for the following ones (of which the first is obvious):

  1. Why should one want to have symbols treated as letters?

  2. The text to be typeset verbatim is absorbed by TeX as an argument delimited by the string \end{verbatim}. In order for this delimiting string to be recognized, both character codes and category codes of its characters must match (see The TeXbook, p. 203, lines -3 to -2). The code that sets up the verbatim environment stores this string with the \catcodes of \, {, and } equal to 12, and with the \catcodes of v, e, r, … ,m equal to 11. If you set the \catcodes of \, {, and } to 11, hence, the delimiting string will no longer be recognized.

The following little program can be used to verify the above claims; try changing the definition of the \MyCatcode macro from

\newcommand*\MyCatcode{12}

to

\newcommand*\MyCatcode{11}

and check that everything works as expected.

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{etoolbox}

\makeatletter

\newcommand*\MyCatcode{12}
\newcommand*\@makemychar[1]{\catcode`#1=\MyCatcode\relax}

\patchcmd{\@verbatim}
    {\let\do\@makeother \dospecials}
    {\let\do\@makemychar \dospecials \@makeother\\\@makeother\{\@makeother\}}
    {}
    {}

\newcommand*{\testverbatim}{%
    \begin{verbatim}%
        \typeout{}%
        \typeout{--------------------------------}%
        \typeout{}%
        \typeout{A few examples:}%
        \typeout{}%
        \typeoutcatcode{\\}%
        \typeoutcatcode{\{}%
        \typeoutcatcode{\}}%
        \typeoutcatcode{\$}%
        \typeoutcatcode{\&}%
        \typeoutcatcode{\#}%
        \typeoutcatcode{\^}%
        \typeoutcatcode{\_}%
        \typeoutcatcode{\%}%
        \typeoutcatcode{\ }%
        \typeoutcatcode{0}%
        \typeoutcatcode{9}%
        \typeoutcatcode{@}%
        \typeoutcatcode{A}%
        \typeoutcatcode{Z}%
        \typeoutcatcode{a}%
        \typeoutcatcode{z}%
        \typeoutcatcode{.}%
        \typeoutcatcode{,}%
        \typeoutcatcode{?}%
        \typeoutcatcode{!}%
        \typeout{}%
        \typeout{--------------------------------}%
        \typeout{}%
}
\newcommand*{\typeoutcatcode}[1]{%
    \typeout{\string\catcode `\string #1 = \number \catcode `#1}%
}

\makeatother



\begin{document}

Non-verbatim text.

\testverbatim
Verbatim text: \{}$&#^_%

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{etoolbox}

\makeatletter

\newcommand*\MyCatcode{12}
\newcommand*\@makemychar[1]{\catcode`#1=\MyCatcode\relax}

\patchcmd{\@verbatim}
    {\let\do\@makeother \dospecials}
    {\let\do\@makemychar \dospecials \@makeother\\\@makeother\{\@makeother\}}
    {}
    {}

\newcommand*{\testverbatim}{%
    \begin{verbatim}%
        \typeout{}%
        \typeout{--------------------------------}%
        \typeout{}%
        \typeout{A few examples:}%
        \typeout{}%
        \typeoutcatcode{\\}%
        \typeoutcatcode{\{}%
        \typeoutcatcode{\}}%
        \typeoutcatcode{\$}%
        \typeoutcatcode{\&}%
        \typeoutcatcode{\#}%
        \typeoutcatcode{\^}%
        \typeoutcatcode{\_}%
        \typeoutcatcode{\%}%
        \typeoutcatcode{\ }%
        \typeoutcatcode{0}%
        \typeoutcatcode{9}%
        \typeoutcatcode{@}%
        \typeoutcatcode{A}%
        \typeoutcatcode{Z}%
        \typeoutcatcode{a}%
        \typeoutcatcode{z}%
        \typeoutcatcode{.}%
        \typeoutcatcode{,}%
        \typeoutcatcode{?}%
        \typeoutcatcode{!}%
        \typeout{}%
        \typeout{--------------------------------}%
        \typeout{}%
}
\newcommand*{\typeoutcatcode}[1]{%
    \typeout{\string\catcode `\string #1 = \number \catcode `#1}%
}

\makeatother



\begin{document}

...

\end{document}
\end{verbatim}

Non-verbatim text again.

\end{document}

With

\newcommand*\MyCatcode{12}

the diagnostic messages read

--------------------------------

A few examples:

\catcode`\\ = 12
\catcode`\{ = 12
\catcode`\} = 12
\catcode`\$ = 12
\catcode`\& = 12
\catcode`\# = 12
\catcode`\^ = 12
\catcode`\_ = 12
\catcode`\% = 12
\catcode`\  = 13
\catcode`0 = 12
\catcode`9 = 12
\catcode`@ = 12
\catcode`A = 11
\catcode`Z = 11
\catcode`a = 11
\catcode`z = 11
\catcode`. = 12
\catcode`, = 13
\catcode`? = 12
\catcode`! = 12

--------------------------------

showing that letters remain of \catcode 11; on the other hand,

\newcommand*\MyCatcode{11}

shows that the verbatim environment is not disrupted, provided that the three crucial \catcodes of \, {, and } are not tampered with.

Addition

For further evidence, try this:

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{etoolbox}

\makeatletter

\begingroup \catcode `|=0 \catcode `[= 1
\catcode`]=2
% Beware:
\catcode `\{=11 \catcode `\}=11 \catcode`\\=11
% ------
|gdef|@xverbatim#1\end{verbatim}[#1|end[verbatim]]
|gdef|@sxverbatim#1\end{verbatim*}[#1|end[verbatim*]]
|endgroup


\newcommand*\MyCatcode{11}
\newcommand*\@makemychar[1]{\catcode`#1=\MyCatcode\relax}

\patchcmd{\@verbatim}
    {\let\do\@makeother}
    {\let\do\@makemychar}
    {}
    {}

\newcommand*{\testverbatim}{%
    \begin{verbatim}%
        \typeout{}%
        \typeout{--------------------------------}%
        \typeout{}%
        \typeout{A few examples:}%
        \typeout{}%
        \typeoutcatcode{\\}%
        \typeoutcatcode{\{}%
        \typeoutcatcode{\}}%
        \typeoutcatcode{\$}%
        \typeoutcatcode{\&}%
        \typeoutcatcode{\#}%
        \typeoutcatcode{\^}%
        \typeoutcatcode{\_}%
        \typeoutcatcode{\%}%
        \typeoutcatcode{\ }%
        \typeoutcatcode{0}%
        \typeoutcatcode{9}%
        \typeoutcatcode{@}%
        \typeoutcatcode{A}%
        \typeoutcatcode{Z}%
        \typeoutcatcode{a}%
        \typeoutcatcode{z}%
        \typeoutcatcode{.}%
        \typeoutcatcode{,}%
        \typeoutcatcode{?}%
        \typeoutcatcode{!}%
        \typeout{}%
        \typeout{--------------------------------}%
        \typeout{}%
}
\newcommand*{\typeoutcatcode}[1]{%
    \typeout{\string\catcode `\string #1 = \number \catcode `#1}%
}

\makeatother



\begin{document}

Non-verbatim text.

\testverbatim
Verbatim text: \{}$&#^_%

% My standard header for TeX.SX answers:
\documentclass[a4paper]{article} % To avoid confusion, let us explicitly 
                                 % declare the paper format.

\usepackage[T1]{fontenc}         % Not always necessary, but recommended.
% End of standard header.  What follows pertains to the problem at hand.

\usepackage{etoolbox}

\makeatletter

\newcommand*\MyCatcode{12}
\newcommand*\@makemychar[1]{\catcode`#1=\MyCatcode\relax}

\patchcmd{\@verbatim}
    {\let\do\@makeother \dospecials}
    {\let\do\@makemychar \dospecials \@makeother\\\@makeother\{\@makeother\}}
    {}
    {}

\newcommand*{\testverbatim}{%
    \begin{verbatim}%
        \typeout{}%
        \typeout{--------------------------------}%
        \typeout{}%
        \typeout{A few examples:}%
        \typeout{}%
        \typeoutcatcode{\\}%
        \typeoutcatcode{\{}%
        \typeoutcatcode{\}}%
        \typeoutcatcode{\$}%
        \typeoutcatcode{\&}%
        \typeoutcatcode{\#}%
        \typeoutcatcode{\^}%
        \typeoutcatcode{\_}%
        \typeoutcatcode{\%}%
        \typeoutcatcode{\ }%
        \typeoutcatcode{0}%
        \typeoutcatcode{9}%
        \typeoutcatcode{@}%
        \typeoutcatcode{A}%
        \typeoutcatcode{Z}%
        \typeoutcatcode{a}%
        \typeoutcatcode{z}%
        \typeoutcatcode{.}%
        \typeoutcatcode{,}%
        \typeoutcatcode{?}%
        \typeoutcatcode{!}%
        \typeout{}%
        \typeout{--------------------------------}%
        \typeout{}%
}
\newcommand*{\typeoutcatcode}[1]{%
    \typeout{\string\catcode `\string #1 = \number \catcode `#1}%
}

\makeatother



\begin{document}

...

\end{document}
\end{verbatim}

Non-verbatim text again.

\end{document}

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