%%
%% The LaTeX Companion, 3ed
%%
%% Example 9-4-4 on page I-680 in "The character scope of math alphabet identifiers".
%%
%% 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}
%StartShownPreambleCommands
\usepackage{amsmath}
%StopShownPreambleCommands

\begin{document}
\begin{align}
    x             &= 12345              \\
   \mathrm{x}     &= \mathrm{12345}     \\
   \mathnormal{x} &= \mathnormal{12345}
\end{align}
\end{document}