Board Thread:General Discussion/@comment-26943411-20150824205605/@comment-25428589-20150829072301

Callofduty4 wrote: I don't think "ex" is a unit of measurement but I may be about to learn something new. Where did you hear about that? Ex is the equivalent of em, except ex changes x-height (the height of a standard, lower case letter), and em changes the width of a standard letter.

See this page on MDN - 1ex = ~0.5em in most fonts (but you have to remember that refers to height, not width like em does).