Guys, the only reason I respond to things like this is not to score points, nor, for that matter, to knock someone off his hobby horse, but because on a folk music site such as this, there are a lot of self-taught musicians or those who have no musical training at all and are trying to muddle through as best they can, and who can wind up hopelessly befuddled when cockamamie ideas like Futwick's are put forth. I'm trying to save them from being let into an impenetrable thicket of screwball ideas such as the thrust of this thread.

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Futwick, the examples of music you linked to are facsimiles of very old music manuscripts, and at least one of them is not written notation, it is tablature. The first two have only four lines, indicating that they predate the current system, which has five lines.   The last one is obviously tablature, but the instrument it's written for is not indicated. Guitar? Lute? Vihuela? The instruments are tuned differently, and if you try to play right off lute tablature on a modern guitar, it isn't going to work.

I could link you to manuscripts in English that were written several hundreds of years ago, using the same letters and the same words (some with the "creative spelling" that existed prior to dictionaries), but due to the style of handwriting, most people these days simply couldn't read them.

Now hear this:

C major and A minor use the same notes. To create a "leading tone," which the natural minor does not have, the seventh degree of the natural minor scale is raised a half-step. Many composers felt that this created too large a gap (a minor third—actually, and augmented second) between the sixth and seventh degrees of the scale, so they took to raising the sixth a half-step to avoid what is a very "Mid-East" sounding interval (this effect is often kept in Flamenco music).

In the key of C, the chords, going scalewise, are C, Dm, Em F, G, Am, Ddim (which is subsumed by playing a G7 chord, and back to C again. Depending on the song or piece of music, any or all of these chords can be used.

C major and A minor (and all other major and relative minor keys) are two different modalities of the same key.

It works for millions of musicians, singers, and composers, and has done so for a couple of centuries now.

If it ain't broken, don't fix it. And it ain't broken!

Don Firth