I have now written a web page about using metric units in typographic
design, summarizing the information that has come up so far in this
discussion:
http://www.cl.cam.ac.uk/~mgk25/metric-typo.html
Please have a look at it, and if you design typographic software, please
please consider whether and how metric font sizes could be supported in
your next version elegantly.
This web page now also contains a table with the DIN 16507-2 preferred
metric font sizes:
Font Size Font Height | Font Size Font Height
[mm] [mm] [pt] | [mm] [mm] [pt]
-----------------------------+----------------------------
1.5 1.1 3.1 | 6.0 4.3 12.2
1.75 1.3 3.6 | 7.0 5.0 14.3
2.0 1.4 4.1 | 8.0 5.8 16.3
2.25 1.6 4.6 | 9.0 6.5 18.4
2.5 1.8 5.1 | 10.0 7.2 20.4
2.75 2.0 5.6 | 12.0 8.6 24.5
3.0 2.2 6.1 | 14.0 10.1 28.6
3.25 2.3 6.6 | 16.0 11.5 32.7
3.5 2.5 7.1 | 18.0 13.0 36.7
3.75 2.7 7.7 | 20.0 14.4 40.8
4.0 2.9 8.2 | 22.5 16.2 45.9
4.25 3.1 8.7 | 25.0 18.0 51.0
4.5 3.2 9.2 | 27.5 19.8 56.1
5.0 3.6 10.2 | 30.0 21.6 61.2
5.5 4.0 11.2 | 35.0 25.2 71.4
(The above mm values are from an old DIN 16507-2 draft. If you implement
metric font sizes, please make sure you get the latest version of the
actual standard yourself!)
Again: The "font size" refers to the baseline distance for which the
font was designed, and is used to generally identify the font. The "font
height" is the actual height of characters such as H or k. The font
height is typically 72% of the font size as a preferred value, but this
is of course left to the discretion of the font designer. One writes
"Courier 6.0" to get the Courier font designed for 6 mm baseline
distance (where the height of an H is typically 4.3 mm or 12.2 pt), and
one writes "Helvetica 6.0/9.0" to get the same font but to use it with
50% more space between the lines. This notation compactly represents the
two most important aspects of font usage.
I have added to the above DIN values in mm also the 72% "font height" in
Postscript points, to make the table easier to digest for those of you
who learned to think in point font sizes. There is a problem however and
you should *not* use that table to blindly convert font sizes between pt
and mm: It is not precisely clear, what actually *is* a say 12 pt font.
Which characters are 12 pt high in a 12 pt font? "H"? "("? In the metric
font world, these things are pretty precisely specified by DIN 16507-2,
but I believe the concept of point-measured font sizes is a bit ill-defined,
as I have seen various conventions in use.
In any case, based on my practical experience in doing digital layout
work, I strongly support the DIN 16507-2 view, that the baseline
distance for which a font has been originally designed is the most
important quantity for doing practical layout work, and that this
distance in mm is what fonts really should be referred to at all levels.
I consider for instance the TeX convention of measuring the height of a
parenthesis in 1/72.27 inch points to identify a font to be much less
practical solution. Decide for yourself what you like most, but please
don't make the error to confuse historical practice with actual
practical convenience.
Markus
-- Markus G. Kuhn, Computer Laboratory, University of Cambridge, UK Email: mkuhn at acm.org, WWW: <http://www.cl.cam.ac.uk/~mgk25/>
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