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What defines the size in a font?

This page is based on Taco's explanation in the list [2018-12-19].

The font designer decides on the ‘natural’ size of the font.

There are two parts to this:

  • What the ‘natural’ size indicates.

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 10 pt, 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 5 pt and 17 pt.

The glyphs in the specific fonts with a smaller ‘natural’ size (like 8 pt) are in fact a little bit bolder and wider than the same glyphs in the font designed to be used at 10 pt. This makes sense when you consider that the 8 pt font is likely be used along with the 10pt font for e.g. footnotes. The 10 pt font used at 8 pt size would look thinner and weaker than the actual font designed for 8 pt.

  • What the ‘design’ size indicates.

Second, a design size in points like ‘10 pt’ is somewhat misleading, because what it actually is, is just a different way of saying “at the expected size for traditional main text”. The “10 pt" is notf necessarily a measure of _anything_ in the font. In fact, font designers sometimes do not use a “XX pt” 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 “10 pt”. As explained above, there are no hard rules. But usually for a modern font the “10 pt" 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).

If two fonts have the same size, is 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."