It's something that's puzzled me; The key that has no sharps and no flats, in essence the "basic" key, is C Major. Well, fine, but why C? Why not label that key and note A, if it's the foundation of the "basic" key of Western music? A second look, however, shows that A is indeed involved; the minor key with no sharps and flats is A minor.
That then leads to the question; does the pitch named A have that name because it is the start of the minor key, and because the minor key was more prevalent in plain chant as of the creation of written music notation? Or is there another reason?