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Re: What's an em

From: Clive Bruton <clive@typonaut.demon.co.uk>
Date: Tue, 1 Feb 2000 19:22:36 -0500 (EST)
Message-Id: <S.0000075842@mail.indx.co.uk>
To: "Erik van der Poel" <www-style@w3.org>, <www-font@w3.org>
Erik van der Poel wrote at 01/02/00 23:24

>It shows a number of different fonts, all rasterized at the same
>"Pointsize", as you call it. The "subjective apparent size" depends on
>the x-height (mostly). Is the "optical size" (as you call it) the same
>as the "subjective apparent size"?
>
>If so, the "Pointsize" isn't the only thing that means anything. CSS2
>claims that the ex/em ratio is important.

X-height is *important*, point size (unless someone really screwed up) is 
*definitive* - subtle difference.

You cannot set type based on measurements of anything else but the point 
size (ie the "body" size), ie look at the x-height on the examples shown, 
you cannot set the line-height as a factor of the x-height. In the 
extreme cases the faces with large x-heights are going to need 
line-heights of (typically) 2.5*x-height whereas those with small 
x-heights are going to need perhaps 8*x-height just to stop acsenders and 
descenders clashing.

If your measurement of line-height is based on point size (ie em or body 
size) at least you can say with some degree of certainty that a 
line-height of 1em will not cause ascenders and descenders to clash.

This debate is totally ridiculous - its like going back to comp.fonts 
five years ago and trying to explain to people that type measurements 
didn't come about out of spite, they were developed over time by people 
who were using the stuff every day.

Those reasons don't change because technology changed a little - it still 
has to be read.


-- Clive
Received on Thursday, 3 February 2000 10:25:40 GMT

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