%%
%% The LaTeX Companion, 3ed
%%
%% Example 11-2-39 on page II-152 in "Fine-tuning tag placement".
%%
%% 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{tlc3examargin}
\pagestyle{empty}
\setcounter{page}{6}
\setlength\textwidth{183.0pt}

%StartShownPreambleCommands
\usepackage[leqno]{amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{gather}   \raisetag{-10pt}
  \text{The sign function: \ }
     \mathcal{S}(x) =  \begin{cases}
                         -1   &  x < 0 \\
                          0   &  x = 0 \\
                          1   &  x > 0
                       \end{cases}   \end{gather}
\end{document}
