# Problem with the subscripts of adjacent sums

I have a problem with the subscript under the sum,

\sum_{i \in X} \sum_{i \in \bar{X}}


for example gives the subscript at different heights for each sum. Does anyone know how to fix this?

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This can be solved using the mathtools package and its command \adjustlimits:

\documentclass{article}
\usepackage{mathtools}
\begin{document}
$\adjustlimits\sum_{i \in X}\sum_{i \in \bar{X}}$
\end{document}


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Vertical adjustments are also possible via \vphantom:
\documentclass{article}
$\sum_{i \in X}\sum_{i \in \bar{X}} \quad \mbox{compared to} \quad \sum_{i \in X\vphantom{\bar{X}}}\sum_{i \in \bar{X}}$