The logo for my website is an illustration from the title page of Theorica Musicae, a treatise on music theory written in 1492 by the Italian priest, musician, and theorist Franchinus Gaffurius. The original drawing depicts not only Gaffurius playing the organ, but also illustrates the medieval system of hexachords, which was central to an understanding of medieval music theory. Modern solfege uses the note names “do-re-mi-fa-sol-la-si-do.” These 8 pitches comprise an octave. Medieval solfege used “ut-re-mi-fa-sol-la.” (“Do” was called “ut.”) These 6 pitches comprised what was called a hexachord, which could be divided into two triplets, ut-re-mi, and fa-sol-la. These two triplets were always divided by a semitone.
Interestingly, the first note of the second triplet could be “re-assigned” as the first note of a new hexachord. Thus, “fa-sol-la” in one hexachord could be considered “ut-re-mi” in a new hexachord. This system of shifting the note names by changing the hexachord was called mutation. When learning a new melody by solfege, if the melody exceeded a six-note range, it became necessary to shift from one hexachord to another, and the “pivot” note would quit the old hexachord and begin the new hexachord.
Click on the illustration above to enlarge it. Then, zoom in further to see the writing on the organ pipes. The note names of a given hexachord are inscribed on a single organ pipe in ascending order. The adjacent organ pipe has the new note names next to the old ones, based on the new hexachord. The third organ pipe illustrates the third hexachord, and so on.
This portrait (presumably of Franchinus Gaffurius) was painted by Leonardo DaVinci, a personal friend of Gaffurius.