%%
%% The LaTeX Companion, 3ed
%%
%% Example 2-4-20 on page I-94 in "showkeys, refcheck --- Displaying \& checking reference keys".
%%
%% 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{231.0pt}


  % small headings for the example
  \makeatletter
  \renewcommand\section{\@startsection{section}{1}{\z@}%
    {-1.25ex \@plus-1ex \@minus-.2ex}%
    {.5ex}%
    {\normalfont\large\bfseries}}
  \renewcommand\subsection{\@startsection {subsection}{2}{\z@}%
    {-1.ex \@plus -.5ex \@minus -.2ex}%
    {.3ex}%
    {\normalfont\bfseries}}
  \makeatother

  % to account for the use of the margin
  \addtolength\textwidth{-80pt}

%StartShownPreambleCommands
\usepackage{amsmath,showkeys}
%StopShownPreambleCommands

\begin{document}
\section{An example}\label{sec}
\subsection{A subsection}\label{unused}
Section~\ref{sec} shows the use of the
\texttt{showkeys} package with a
reference to equation~(\ref{eq}).
\begin{align} a &= b \label{eq}  \\
              a &< b \label{eq2} \\
              a &> b     \end{align}
\end{document}
