%%
%% The LaTeX Companion, 3ed
%%
%% Example 2-2-1 on page I-35 in "Numbering headings".
%%
%% 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{207.0pt}


% small headings for the example
\makeatletter
\renewcommand\section{\@startsection{section}{1}{\z@}%
                                    {-1.25ex \@plus-1ex \@minus-.2ex}%
                                    {.8ex}%
                                    {\large\bfseries}}
\renewcommand\subsection{\@startsection{subsection}{2}{\z@}%
                                    {-1.25ex \@plus-1ex \@minus-.2ex}%
                                    {.5ex}%
                                    {\normalsize\bfseries}}
\makeatother


%StartShownPreambleCommands
\renewcommand\thesection{\Alph{section}}
%StopShownPreambleCommands

\begin{document}
\section{Different-looking section}
\subsection{Different-looking subsection}
Due to the default definitions not only the
numbers on sections change, but lower-level
sectioning commands also show this
representation of the section number.
\end{document}