Having had a really insightful thread on Autumn Leaves - that was hugely helpful. It brought together lots of abstracts for me.
I then started to think about keys - so I considered Heavens door 'G C D' thats 'I Vi V' and following that made sense of some of slashes passages in the solos as he follows the chords.
So! I then started to think about something else I've been playing with on my new Hemicaster. A Cadd9 G D Dsus4 D. A nice little chord progression. Deffo starts on A, deffo returns to A. Must be in A then
But it can't be because the chords in this key are A major, B minor, C# minor, D major, E major, F# minor, and G# diminished.
I looked that up :-)
So Amaj is in key, C isn't, G isn't but D is. But its in A?
How does one approach this?
Comments
Also, chord progressions don't always start with "main"/first scale degree chord I e.g. you could have (quite common) chord progression of
II – IV – V
So, above progression in the key of G would be: D -C -A (look ma, no G ;-) )
edit:
you should study/read some articles about tonic/tonal/pitch center. I don't have any links, but googling it will give you enough links.
Supportact said: [my style is] probably more an accumulation of limitations and bad habits than a 'style'.
The use of the chord on the minor 3rd - the C - is a “flat 3” chord or bIII. Also very common. Check out Smoke on the Water for another example.
The reason these two chords are so often used is because blues rock frequently blends major and minor together. A lot of it is agnostic of the major and minor moods. The key of A minor would have a C chord and a G chord, yet the A chord has a major 3rd (or is even just a power chord with the 3rd lacking). You can kinda think of your progression as being “in A” rather than as being “in A major”.
Supportact said: [my style is] probably more an accumulation of limitations and bad habits than a 'style'.
Lots of blues/rock has this minor/major ambiguity thing going on that allows use of a minor scale over a major chord.
You'll often come across blues/rock that uses the major chords based on the relevant pentatonic minor scale. For instance the A minor pentatonic consists of the notes A, C, D, E, and G. So the chord progression you describe which is essentially A, C, G, D (all major chords with roots from A minor pentatonic scale) means you could solo over it using notes from the pentatonic minor scale.
I don't feel I've explained that particularly well, but hopefully you can see what I'm trying to get across.
Edit - Thinking about this further, I guess the reason this works is (looking at major chords with roots based on the A minor pentatonic scale)...
A major contains two notes from the scale (A & E)
C major contains three notes (C, E, & G)
D major contains two notes (D & A)
E major just the one (E)
G major contains two (G &D)
...so there are plenty of 'right' notes in there to make it work.
If I saw Hey Joe written out using musical notation I'd expect to see four sharps, so I'd say that it's in E despite there being plenty of chords that wouldn't normally be considered to be in that key.
Except for the C, the song is built from major chords whose roots are based on the E minor pentatonic (see my ramblings above about how these chords form the building blocks of a lot of blues/rock).
As has already been pointed out, there is nothing wrong with borrowing notes or chords from other keys.
Supportact said: [my style is] probably more an accumulation of limitations and bad habits than a 'style'.