%%
%% The LaTeX Companion, 3ed
%%
%% Example 2-2-18 on page I-49 in "Conditional heading layouts".
%%
%% 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{135.0pt}

% For the book example:
\setlength\textheight{8\baselineskip}
\sloppy

%StartShownPreambleCommands
\usepackage{titlesec}
\titleformat{name=\section}[block]
  {\normalfont\bfseries}{\thesection.}{6pt}{\filright}
\titleformat{name=\section,numberless=true}[block]
  {\normalfont}{---}{12pt}{\itshape\filcenter}
%StopShownPreambleCommands

\begin{document}
\section{A Head}
Some text to fill the page. Some text to fill the page.
\section*{Another}
Some text to fill this line.
\end{document}
