Should there be a difference between the glyphs of superior numbers and ordinal ones? In some fonts I see that they are set in different lookups, but the glyphs are exactly the same (better: the same double glyphs with different extensions). I instead created only one lookup with two subtables, one with ordn and one with sups with reference to the same glyphs. What is the most correct practice? Thank you
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Personally, I think supporting old-fashioned typography is sometimes appropriate to the design of a typeface.
Not only is your approach more efficient, but it also reduces the likelihood of error by only having one set of data. Yet... given the small number of glyphs typically involved in 'sups' and 'ordn', I suspect the file size savings is just a handful of bytes. So, an efficiency, but not a terribly important one.
Since, however, some fonts have both types of numbers, I went to see, as I wrote above, the EB Garamond: it contains two sets of absolutely identical glyphs, one for superscript and one for ordinal numbers. I wondered if in general they must be the same or if it is more correct to distinguish them typographically, and in what way: the ordinals even smaller? And higher?
Are you sure that's the distinction? It may be one set for superscripts and another set as fraction numerators.
I put superior letters in the <ordn>feature, and, if a full alphabet, code them for English (just like Word!—but not as default).
I often make two sizes of superior figure, with those for <sups> being clearly smaller than those for <frac> and <numr>.
2 last questions:
1) I saw that different fonts present different solutions, but is it appropriate or not that the superscript numbers (those that indicate the footnotes, for instance) are the same as those that make up the fractions? In some fonts they are the same size, in other fonts the footnotes numbers are smaller ...
2) Do superscript letters and numbers have the same baseline? In GaramondPremierePro the don't have, in EBGaramond they do. Is there a standard, or are there some reasons of opportunity to marry one or the other solution?
My ordinal indicators hang from the cap height, and share baseline with the superiors. They are scaled proportionally from the lowercase letters and lining figures, so the superior figures stick up above the cap height slightly.
I have always designed the tops of superscript figures to align with the top of Capitals, but I now incline more to align them with the top of ascenders to make footnote references more visible when scanning a line of text.
One could also align the top of superscript ascenders with the top of ascenders, and the bottom of superscript figures with the bottom of superscript letters.
I always design all my superscript (and subscript) on the assumption that they should share a common baseline and, regardless of the actual set included in a particular version of a font, I proportion them so that they can be reasonably extended to full uppercase, lowercase, numeral and common punctuation and math symbol sets.
This means, of course, taking lowercase descenders into account when determining the size and alignment height of the superscripts.
Scientific inferiors (0-9) also reuse the superscript glyphs as composites, and bisect the baseline.
Note that the superior figures are smaller than the subscript and fraction figures. I make the subscripts larger, because they are often read as part of immersive text, not merely for reference.
Aside from the choice of making them slightly different in size, by "subscript" you mean the superscript/subscript (onesuperior-u00B9, oneinferior-u2081 et al.), which you make the same height of figures which will compose fractions i.e. numerators and denominators (one.numr, two.numr - one.dnom, two.dnom)? Thanks much!