So a lot of people can tell you the optimum line length for reading text is, 63 to 72 characters.
We know this is not a 'fixed' number for all readers and all compositions, but is this a fixed value for all fonts?
No... clearly it refers only to text faces.
What's a text face?
Good question! Let's assume it is a face that is in a particular range, numerically, in its relationship between primary stem thickness, and alphabet length-to-cap height, (or l.c. ht. to alphabet length)
Now, can one assume all faces fitting into that mathematical description of 'text' are good at 63-72 characters per line?
Perhaps... but then what are the best line lengths for non-text faces, if there is such a length-per-design?
Is that one number of characters per line actually hiding a continuum waiting to be applied to all alphabets?
Now I'm tired of talking to myself, does anyone know of any studies, theories or ideas of their own? :-)