The key of C is arbitrary (you could rename any of the other keys to C and rotate the whole system, and everything would work the same). But there's a case to b

The key of C is arbitrary (you could rename any of the other keys to C and rotate the whole system, and everything would work the same). But there's a case to be made that the diatonic major scale (Ionian mode) has a certain primacy in modern Western music, certainly more than say, Phrygian or Locrian. Both listeners and composers tend to hear the major scale as the most default, vanilla, un-surprising scale there is. It's one of the things that gives certain tunes like "Twinkle Twinkle Little Star" and "Happy Birthday" an almost annoyingly predictable quality. Even something pretty tame like Mixolydian sounds a little surprising compared to the major scale. So it's legitimate to ask, if we're going to letter our notes based on the notes of one key, why not pick the key that sounds the most basic and fundamental? (The answer is that at one time folks heard music in a different way and Ionian didn't have the priority that it does now; see my other comment [1]). It also probably doesn't hurt that Aeolian (natural minor) is arguably the second most natural and "default" sounding mode, so it's not nearly as weird as if we decided to letter the notes off of Locrian or something. [1] https://news.ycombinator.com/item?id=12529437 https://news.ycombinator.com/item?id=12529437