Just some random thoughts to muddy up the water a bit.
Relative major and minor -
This simple concept causes a lot of confusion about major and minor keys.
The 101 level theory book usually says something to the effect of starting the major scale on the 6th degree gives you the minor scale. They share the same key signature etc, etc.
This leads to the type of questions we see in this thread. Why bother to call C the major key and A the minor key if they're just the same notes and chords?
I'll leave the history lessons for JonR. Let's just say that early composers discovered an interesting phenomenon. You can take a group of Notes and/or chords and by how you arrange them, you can get the listener to hear different notes/chords within the group as being the main or most important note/chord. We'll call this most important note/chord
Tonic (fancy way of saying "the tone").
Let's look at the chords that come from the C major scale (called
diatonic which is a fancy way of saying "using only notes from the scale"):
C Dm Em F G Am Bdim
Of all these chords, the C is the easiest to make sound like the tonic. If you play a simple progression like so:
C Am F G C
There isn't really any question that C is home base for the progression. The music sounds like it comes to rest on the C chord.
Composers discovered that, without much effort, you can also make the Am chord sound like home base. Try the same progression again, but this time end on Am:
C Am F G Am
You may not hear the music come to full stop like ending on the C chord, but there's some confusion. It sounds like maybe A minor is home base. And that's not doing anything but changing the chord you end on. If you start mixing things up bit, you can strengthen the sound of Am as tonic:
C Am F Em Am
If we leave out the C chord altogether, you can really start to hear the Am as tonic:
Am F G Am
Am G F Em Am
Am Dm Am Em Am
This tendency for either C or Am to sound like the tonic has sponsored a lot of music that modulates (shifts the tonic emphasis) between the relative major and minor. It's very common to start a piece off based on the major, shift to minor part way through and shift back to major for the end. The opposite is also common... start minor, shift to major, end minor.
Try this:
C F G C - C F G C - Am Em Am Dm - Am F G C
You can make other chords in the group sound like the tonic as well, but it takes a little more work to do so. The C and Am just seem to be made for the job more than the other chords.
This sponsored the concept of
keys with the minor key being built on the 6th degree of the major key and both sharing the same key signature.
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Now, once you have the idea that the tonic can be either major or minor, you start seeing them as separate entities. One way of doing this that composers discovered is modulating to the parallel minor instead of the relative minor... starting on C major, shifting to C minor, shifting back to C major.
Composers also began adding diatonic and non-diatonic notes to the harmony and melody to further strengthen the major and minor tonic. In the major key, adding an F note to the G chord (G7 chord) seemed to push that chord towards the tonic (C chord). In the minor key they discovered the same tendency by changing the G note to G# and adding the D note to the E chord (E7). In parallel this simply means that you can use a G7 chord to set up either C major or C minor.
In classical theory language this is presented as:
Any note can be made to sound like the tonic. This establishes the key.
There are two common modes/scales used to determine the tonality of the key... major and minor.
Although there are a few common major and minor scales that are used more than others, any mode/scale/chord can potentially be used within a key without weakening the sense of tonic.
This includes using the entire chromatic scale within a given key.
A brilliant example of this is Rimsky-Korsakov's "Flight of the Bumble Bee" where he takes a melody that is pretty much a chromatic frenzy and grounds it in solid minor key harmony.
https://www.youtube.com/watch?v=6QV1RGMLUKE