Are there specific rules on which letters (miniscules and majuscules) are shaded?
@signcarver Yes, at least initially.
You may find the following interesting.
The first three comprise a section of the
Spencerian Key to Practical Penmanship , 1866 by H. C. Spencer et. al. only the a was sometimes shaded among the 1-X height letters, the ascenders and descenders being mostly shaded as shown in the fourth scan, from
The New Spencerian Compendium, 1876, Spencer brothers. You can also see how the caps and numbers were shaded here.
As time went by and Spencerian became more ornamental, more miniscules were/could be shaded as shown by Michael Sull"s
Learning to Write Spencerian Script , 1993. Note that the "new" shades are more delicate than the ascender/decender shades--see m, n, v, b for example, in Sull's exemplar.
I've seen every letter shaded, except maybe i and e, in some ornamental Spencerian examples.
Unless you are writing shaded Spencerian, a modest use of shading of x-size letters looks best to my eye (that's a personal view), and avoiding putting two shaded miniscules in a row.
Use exemplars from the masters as a guide and to your taste.