[Tex/LaTex] Different behavior of setlength and settowidth

Question

In this example, I would expect all "MMM" to be well-aligned to each other. But they aren't:

\documentclass{article}%
\usepackage{amsmath}%
\newcommand{\before}{mmm}%
\newcommand{\after}{MMM}%
\newlength{\myplus}%
\begin{document}%
\begin{multline}%
\settowidth{\myplus}{\before}%
\the\myplus\hfill\\%
\before,\after\hfill\\%
%
\hspace{\myplus},\after\hfill\\%
\hspace*{\myplus},\after\hfill\\%
%
\setlength{\myplus}{25pt}%
\hspace{\myplus},\after\hfill\\%
\hspace*{\myplus},\after\hfill\\%
%
\hspace{25pt},\after\hfill\\%
\hspace*{25pt},\after\hfill\\%
\end{multline}%
\end{document}

In particular, \mylength and 25pt behave differently even after \setlength{\myplus}{25pt}. Also, settowidth sets the width to 25pt, but \myplus acts even different after that.

Why is that, and how can I achieve spacing in combination with setlength?

Answer 1

You have set the length in a local group, so it is 0pt again on the following line, also you have set it to the width of mmm in the text font, but show it set in math italic,

perhaps you want

\settowidth{\myplus}{$\before$}%

outside (before) the multline, depending on your real use case.