On Ems  

I’ve always had trouble wrapping my mind around how ems work when working with typography on the web. I’ve been reading through this web typography guide and it has this explanation that makes it super clear how measuring in ems works:

[t]he em is a sliding measure. One em is a distance equal to the type size. In 6 point type, an em is 6 points; in 12 point type an em is 12 points and in 60 point type an em is 60 points. Thus a one em space is proportionately the same in any size.