\mathstrut ¶Synopsis:
\mathstrut
The analogue of \strut for mathematics. See \strut.
The input $\sqrt{x} + \sqrt{x^i}$ gives output where the
second radical is taller than the first. To add extra vertical space
without any horizontal space, so that the two have the same height, use
$\sqrt{x\mathstrut} + \sqrt{x^i\mathstrut}$.
The \mathstrut command adds the vertical height of an open
parenthesis, (, but no horizontal space. It is defined as
\vphantom{(}, so see \phantom & \vphantom & \hphantom for
more. An advantage over \strut is that \mathstrut adds no
depth, which is often the right thing for formulas. Using the height of
an open parenthesis is just a convention; for complete control over the
amount of space, use \rule with a width of zero. See \rule.