In LaTeX, the
ex unit represents the height of a lowercase 'x' in the current font.
Is there a length in LaTeX that represents the height of a capital 'X'?
The information about 1ex is stored in the font; it is usually the height of a lowercase ‘x’, but it need not be necessarily.
You can access the height of an uppercase ‘X’ by
A way for expressing lengths in this ‘unit of measure’ is
so you can say something like
\mylen has been allocated with