# Re: stability of root em unit spec

From: Tab Atkins Jr. <jackalmage@gmail.com>
Date: Wed, 7 Jan 2009 09:53:20 -0600
To: "Philip TAYLOR (Ret'd)" <P.Taylor@rhul.ac.uk>
Cc: "Mikko Rantalainen" <mikko.rantalainen@peda.net>, www-style@w3.org, "Håkon Wium Lie" <howcome@opera.com>
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On Wed, Jan 7, 2009 at 8:38 AM, Philip TAYLOR (Ret'd)
<P.Taylor@rhul.ac.uk> wrote:
> Mikko Rantalainen wrote:
>
>> I believe that it's only a coincidence that existing lengths are
>> two-letter.
>
> I really do not accept that the statistics support this hypothesis.

"in", "cm", "mm", "em", "ex" and "px"

Well, it's an accident of history and momentum, then.

"in" is lifted straight from English abbreviations, where compactness
is an important quality because they were meant to be *written*.  We
made the abbreviations *one* letter long when possible, but a single
"i" is probably too easily mistaken for a 1 in handwriting.  Had
"miles" been a useful CSS unit, we'd likely see "m" naming it.

"cm" and "mm" are straight from the metric system, which is composed
(almost) completely of one-letter unit abbreviations and one-letter
magnitude abbreviations.  This is nothing more than the minimum
necessary to specify a wide range.  The fact that it is two letters is
nothing special or significant - a handful of metric units require
more than one letter to abbreviate (within the base units, that would
be the mole, abbreviated "mol", and the candela, abbreviated "cd").
Again, it's an issue of compactness of notation.  Had a base metric
length unit been useful for CSS, we'd see a one-letter unit name
instead (that's exactly what we do have for seconds, written as "s").

"em" and "ex" are two letters because, well, that's how you spell the
spoken names of "M" and "X".  Most english letters are named by
extremely simple sounds which can be spelled with two letters.  Some
aren't, however (and in other languages the letters often have longer
names).

"px" appears to be the only one that might be genuinely attributable
to historical momentum, but it can be just as easily attributed to the
fact that "px" is the smallest you can unambiguously abbreviate the
word.  However, it was also often written as "pel".

I have no true problem with root-em being abbreviated as "rm", I just
think it obscures the meaning of the element.  "rem" clearly links it
to "em", and is hardly long or strange.

Shifting the discussion a moment...
On Wed, Jan 7, 2009 at 9:53 PM, Håkon Wium Lie <howcome@opera.com> wrote:
> I'm still working on this draft -- adding gr and fr units and such.
> The draft will be announced on this list when it's ready for review.

Are there any plans to stabilize the name of the flex unit across all
CSS modules?  You use "fr" (short for 'fraction'), and I know the Grid
Positioning and Template Layout modules use "*".  There's also an
argument for using "fl" (short for 'flexible'), though I don't believe
that usage is attested in any current modules.

I'm personally against "fr" as I feel that "fraction" or "fractional"
is somewhat opaque to the actual operation of the unit, but I'm mainly
just interested in seeing a consistent name be established.

~TJ
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Received on Wednesday, 7 January 2009 15:53:58 UTC

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