Re: what defines the font size?

[Taco Hoekwater <taco@elvenkind.com>]

Hi Pablo,

Let me go back an try to answer the original question:

> On 15 Oct 2018, at 21:21, Pablo Rodriguez <oinos@gmx.es> wrote:
> 
> Dear list,
> 
> sorry for this basic question, but what defines the size in a font?

The font designer decides on the ‘natural’ size of the font. 
There are two parts to this.

First, what the ‘natural’ size indicates is the designer’s 
_intended use size_ for the font, such that when you plan
to use the font “Times-Roman” without any special rescaling,
it should in fact be equivalent to “Times-Roman at ’natural size’”.

For most fonts, this ‘natural size’ is 10pt, but special display
or footnote fonts may have a different intended use size, and the 
font designer may have made special glyph adjustments for that
purpose. For example, the computer modern family has special fonts 
with a ‘natural’ size anywhere between 5pt and 17pt. The glyphs in
the specific fonts with a smaller ‘natural’ size (like 8pt)
are in fact a little bit bolder and wider than the same glyphs in 
the font designed to be used at 10pt. This makes sense when you
consider that the 8pt font is likely be used along with the 10pt 
font for e.g. footnotes. The 10pt font used at 8pt size would look 
thinner and weaker than the actual font designed for 8pt.

Second, a design size in points like ‘10pt’ is somewhat misleading,
because what it actually is, is just a different way of saying “at
the expected size for traditional main text”. The “10pt" is not 
necessarily a measure of _anything_ in the font. In fact, font designers 
sometimes do not use a “XXpt” design size at all.The Minion font family 
has fonts with names like "Minion Pro Caption" and "Minion Pro Display”,
which is actually a better indication of the information the font
designer wants to convey.


That leaves the question of what the actual size is of a font used
at “10pt”. As explained above, there are no hard rules. But usually
for a modern font the “10pt" is the _vertical_ space needed to enclose 
all of the ascenders and descenders in the font when all the glyphs
are overlaid on top of each other. Traditionally, this was also the
with of an ‘em’, going back to the Roman era, where inscribed text fitted 
characters into a square. But these days that is no longer always the 
case, since some font families have condensed or extended members 
(and it really only applied to ‘upright’ fonts anyways).

In short:

> If two fonts have the same size, I think there may be a dimension which
> has the same length in both. Which one is this?

No, there is no such thing. "TeX Gyre Bonum and TeX Gyre Adventor at twelve 
point” really only means this:

  "TeX Gyre Bonum at a somewhat larger size than the TeX Gyre Bonum designer 
  intended and TeX Gyre Adventor at a somewhat larger size than the TeX Gyre 
  Adventor designer intended"

Nothing more, nothing less.

Best wishes,
Taco




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