More ... part 2.
Chet is using the C Major scale over a chord progression so that chord progression must surely be in the key of C right?
Well, yes, of course.
Here is the C Major scale in a linear notation with a repeat so it spans two octaves.
R 2 3 4 5 6 7 R 2 3 4 5 6 7 R
C D E F G A B C D E F G A B C
The chords in the progression are all made from notes in the C Major scale:
C = C, E, G
Dm = D, F, A
G7 = G, B, D, F
Am = A, C, E
If you do not already know about simple chord construction then look carefully at each group of notes within the chords, then find those same notes as laid out in the line of notes in the C Major scale.
There should be something you notice ... a pattern, a sequence, a common feature that they share.
Are you still struggling to see it?
If so, look at this ...
C = C, E, G
R 2 3 4 5 6 7 R 2 3 4 5 6 7 R
C D E F G A B C D E F G A B C
Dm = D, F, A
R 2 3 4 5 6 7 R 2 3 4 5 6 7 R
C D E F G A B C D E F G A B C
G7 = G, B, D, F
R 2 3 4 5 6 7 R 2 3 4 5 6 7 R
C D E F G A B C D E F G A B C
Am = A, C, E
R 2 3 4 5 6 7 R 2 3 4 5 6 7 R
C D E F G A B C D E F G A B C
Do you now see how the very make up and structure of those chords stem directly from notes of the C Major scale, notes that appear in the C Major scale at regular spacing / intervals?
Yes?
Fantastic.
These regular intervals taken from the scale are called 'stacked 3rds' because the chords are made up from a simple pattern thus:
(for each given start note) use the 1st note, skip the next note - the 2nd - then use the 3rd note. The 3rd note becomes the new 1st, skip the 2nd, use the 3rd. Stacked 3rds. All built on notes taken from the same scale.
Does this make sense?
If so you are beginning to connect the C Major scale with chords in the key of C. And you are beginning to see something fundamental about chord construction itself.