Vertical alignment of overline

Question

I would like to have the horizontal lines aligned vertically in next document

\documentclass{standalone}
\begin{document}
\Huge
$\overline{z\,z'} = \overline{z} \, \overline{z'}$
\end{document}

The bar over z is lower than the others.
With \vphantom

\documentclass{standalone}
\begin{document}
\Huge
$\overline{z\,z'} = \overline{\vphantom{z'}z\,z'}
  = \overline{\vphantom{z'}z} \, \overline{\vphantom{z'}z'}$
\end{document}

the two bars on the right are vertically aligned but they are not with the first bar.

1. Why are the rightmost bar too high?
2. How can I get the same height as the first?

Answer 1

Your \vphantom{z'} is making a strut that is taller than the z' as superscripts under \overline are set in cramped style.

LuaTeX has a primitive to force cramped style but otherwise mathtools has a macro \cramped that forces essentially the same layout.

\documentclass{standalone}
\usepackage{mathtools}
\begin{document}
\Huge
$\overline{z\,z'} = \overline{\vphantom{\cramped{z'}}z\,z'}
  = \overline{\vphantom{\cramped{z'}}z} \, \overline{\vphantom{\cramped{z'}}z'}$
\end{document}