Re: What's an em (was RE: Units, font sizing, and zoom suggestion for CSS 3)

Karlsson Kent - keka wrote at 31/01/00 17:21

>	An em was originally the width of an M (swashes don't count).

Sorry, I think that is (mostly) just a coincidence, an em is a square (ie 
it has two dimensions, not one) and is defined by the *body* size of a 
piece of movable type. It is scalable since it relates directly to the 
size of type in question, an exception to this is that in general people 
measuring by "ems" mean "pica ems" which are 12pt ems (though this 
doesn't apply in the CSS model).

>	In CSS an em is the "font-size", and relatively few use it as yet.
>		I haven't seen any good argument for keeping it "as is",
>		other than an unwillingness to correct a mistake and/or
>		to align with TeX. (Call it 'wem', or 'tem' if you like,
>		but then deprecate the current 'em'.)  
>
>		Correcting the definition of an em to what it 1) usually
>		means (width of an M), and 2) what it means (width of an M)
>		in *the* other *major* electronic typesetting context (TeX)
>		should have little impact for most.  If someone asks for a
>		line-height of 1em, then they should get what they ask for,
>		a line height the same as the width of an M, nothing else.

This makes no sense. In your argument the line height would have no 
relationship whatsoever to the size of the type. Conceivably if one were 
setting line height for a condensed face it would have to be set in 
excess of 2.0em, on the other hand for an expanded face it could be as 
little as 0.5em. In both cases the perceived line height would be the 
same.

And how would you specify line-height when you mixed different width 
fonts on a single line, this would happen even if you were setting common 
combinations like bold and italic styles!?

The measurement of line-height or em size needs to be explicitly linked 
to a typeface's size, not its width.


-- Clive

Received on Monday, 31 January 2000 17:08:17 UTC