Simple questions, should I name my small caps with an upper or lower case character ?
a.smcp or A.smcp
They should be named according to what base character they represent. So in the most common situation where the small cap glyph is being substituted for "a", you should call it "a.smcp".
Here's the deal: For the document you're preparing, the use of small cps is a mater of style. It doesn't mater what you call them. But the *file* you're preparing may have other uses. And the possible PDF you're preparing may have the text extracted for other use. With a PDF, anything after the period can be lost. In that case, should the letter be upper or lower case? By having both A.sc and a.sc, everything is resolved.
In spite of that, I tend to use A.sc only. I work in a fairly limited environment: books only. In that kind of work, when a style uses small capitals, replacement by a full cap is almost always acceptable, though may not be preferable. Replacement by a lower case letter can be just wrong.
It is not too much extra work to have both. Having both does use up some of the allowable 65,000 kerns possible...
& BTW, since there is a limit to the number of characters you can have as names for glyphs in a font, if you're planning a large number of glyphs, using "sc" instead of "smcp" saves a few...
What about using "a.smcp" for the basic "small capitals" feature and "A.c2sc" for "capitals to small capitals"?
Tim, that's exactly what we do if a client insists that text be searchable and extractable from PDFs that have been distilled from print streams (and hence rely on Acrobat parsing of glyph names to reconstruct text).
We have other clients who don't care about this, and are happy to have only one set of smallcap glyphs, in which case I tend to call them something like /A.sc/ or in some older fonts /A.small/.
And then we have one client who actively doesn't want to make fonts compatible with Acrobat in this way, like their TTFs to have format 3 post tables, which contain no glyph names.
My recommendation is the following:
1. If you decide to duplicate the small caps glyphs, i.e. to have separate sets of identical small caps glyphs, one to work with the "smcp" feature and one to work with the "c2sc" feature, then I recommend naming the glyphs intended for "smcp" using lowercase names, i.e. "a.smcp", and the glyphs intended for "c2sc" using uppercase names i.e. "A.c2sc". This gives you a very clean distinction.
2. If you decide to include just one set of small caps glyphs, the I think using uppercase names is better because those glyphs will resolve to all-uppercase text strings, and those are better than all-lowercase text strings (i.e. "I had an IBM machine an an OLIVETTI machine" is better than "I had an ibm machine and an olivetti machine"). In this case, I'd name the glyphs either "A.smcp" or "A.c2sc". In fact, these days I'm leaning towards "A.c2sc" as the generic name for small caps, because that makes standardizing glyph names easier.
In other words, your basic set of small cap glyphs could be named "A.c2sc" etc., and, should you choose to include a duplicate set, that set could be named "a.smcp".
I think this is the "cleanest" and most systematic solution.