The cap M is only 1 em width in a very few fonts. Even fewer will have the cap N exactly 1 en in width.

The em is a horizontal dimension that varies with typesize. It is defined as being the same size as the pointsize of the face. Thus a em is 12 points wide in a 12 point font, but 24 points wide in a 24 point font. This makes it especially good for specifying paragraph indents, among other things.

(Also, when you are measuring a font, remember that you must measure the ascender, body, and descender space, and even then there might be a small additional amount of "side-bearing" white space around the characters.)

Perhaps History. I remember asking the owner of a shop that used Linotype Linecaster's -- which don't use an em system -- how wide an em-space was. "Why," he said, "the width of a capital 'M'".

Personally, I just don't remember just what got put in the magazines . . . We've all been working with em-based systems for so long.

I agree – it's not going anywhere. Ironically, the em is also widely used as a unit of type for CSS.

Marc, that's a very old-fashioned concept.
And quite possibly it was never really true.

BTW, I have that book... in Spanish! :-)
It's a very useful work I think.

hhp

Thank's a lot guy's !!!

Marc, with 1000 units per em, Adobe Garamond's M is 912 units wide, and Adobe Caslon's M is 981 wide.

Traditional seriffed M's did have approximately square bodies, hence the name for a body width space, which is square, is an 'em quad', and we continue to talk about the 'em' in digital type. For more than you ever wanted to know, see this thread: http://typophile.com/node/27742

Thanks William,

BTW, I would like to read the EULA in french, do you know where a can find it ?

>the EULA

There are as many EULAs--End User License Agreements--as there are software publishers, I think. Among other places, you will certainly find French language EULAs at the sites of French type founderies, such as this one from the Typofonderie of J-F Porchez.