# settowidth in substack

I am aware of Align in Substack, but I am trying another solution and would like to understand why it is not working:

\documentclass{article}%
\usepackage{amsmath}%
\newlength{\mylength}%
\newcommand{\before}{k_\rho k_\rho k_\rho k_\rho}%
\begin{document}%
\LARGE
\begin{align}%
%\substack{% <-- uncomment this line
\before=something\\%
\settowidth{\mylength}{\ensuremath{\before}}%
\hspace{\mylength}=something\\%
%}% <-- uncomment this line
\end{align}%
\end{document}%


First of all, alignment is working fine in a multline or align. But in a substack, it is not, the = are not aligned.

• \settowidth{\mylength}{$\before$} does the same, by the way. – bers Oct 23 '14 at 18:07

It is the same as comparing $X_{a=b}$ with $X_{\mbox{$a=b$}}$ If you start a new math expression inside a box it will be textsize even if you are in a subscript. Explicitly adding \scriptstyle gets you back on track.

\documentclass{article}
\usepackage{amsmath}
\newlength{\mylength}
\newcommand{\before}{\scriptstyle k_\rho k_\rho k_\rho k_\rho}
\begin{document}
\LARGE
\begin{align}
\substack{% <-- uncomment this line
\before=something\\
\settowidth{\mylength}{\ensuremath{\before}}%
\hspace{\mylength}=something\\%
}% <-- uncomment this line
\end{align}
\end{document}

• Awesome, thank you. I had suspected something like this, but my test with \LARGE worked as expected (LARGE font size was respected in settowidth). I was not aware there is a difference between text and subscript other than font size that might not be respected implicitly. – bers Oct 23 '14 at 18:41