Do CSS ems always represent the font size?

Question

From what I know, the em keyword in CSS means the current size of a font.

So if you put 1.2 em, it means 120% of the font height.

It doesn't seem right though that em is used for setting the width of divs etc like YUI grids does:

margin-right:24.0769em;*margin-right:23.62em;

Everytime I read about em, I forget what it really represents.

I'm hoping someone can explain it to me so it sticks in my head heeh.

Answer 1

Historically it is the width of an "M" in the font. Hence the name! In CSS2.1 it is defined to be the same as the font-size.

In many cases it seems more natural to use em rather than points or pixels, because it is relative to the font size. For example you might define a text-column to have a width of 40em. If you later decide to change the font-size, the column will still keep the same number of letters per line.

Answer 2

Traditionally, em is the width of the upper case M. In practise though, an em is the point size of the font.

em dash versus en dash.