This diagram on fol. 122r of Ms. Codex 1248 is called a Guidonian hand, a tool that was developed to help medieval singers learn how to sight-sing. Looking closer at this hand can teach you quite a bit about how the names of notes have changed since the medieval period, and which ones have been added to the hexachord. And, as a bonus to go along with it all, there is also a very fancy cuff.
Manuscript description and digital images can be found here at OPenn.






















