Music Theory
For my magic system, I spent several weeks pushing myself through a crash course in music theory.
My magic system is split into two main types of music: conjuring and manipulation.
For conjuring, I used the circle of fifths as a table for different types of conjured magic. Each key (and the relations between them) has ties to specific elements. This is the foundation of magic.
Manipulation takes this a step further. For context, everything and everyone in the world of my book has a song personal to them. Manipulation uses these preexisting songs, and alters them by using modes. For this type of magic, musicians play whatever song they want to manipulate in 5 of the existing 7 modes (Ionian and Aeolian are for conjuring). Each of these remaining modes (Lydian, Mixolydian, Dorian, Phrygian, and Locrian) manipulate the song in a different way, and by extension, the object they belong to.