Originally Posted by: l.pearson1825

I have a scale question. I have been trying to use the major scale more and I have a simple progression of Am, C, G, Am.

[p]Those chords could be in 2 different key signatures: A minor/C major (relative major key), or G major/E minor. C major & A minor contain the same notes they are essentially the same thing in this situation being just relative major & minor of each other.

You figure out the key signature by writing out or considering all the notes in all the chords being used, put them in order to see what scale they make.

Am: a-c-e

C: c-e-g

G: g-b-d

a-b-c-d-e-g (A minor scale missing the note F)

c-d-e-g-a-b (C major scale missing F)

g-a-b-c-d-e (G major scale missing F#)

Because your chords don't specify F or F# it could either. You can use either & see which you prefer. Or you can just play either of those & avoid the F or F# all together.

So you have these options:

Key of A minor:

Am (i) / C (bIII) / G (bVII) / Am (i)

Key of C major:

Am (vi) / C (I) / G (V) / Am (vi)

Key of G major:

Am (ii) / C (IV) / G (I) / Am (ii)

It sounds like A minor to me because you start &, more importantly, end on that chord. But it could be any of them depending on the melody you play or sing over those chords.

I cover the process of figuring out the key signature & then learning how to use it to play solos in this collection of tutorials on improvisation.

https://www.guitartricks.com/collection/learning-to-improvise

Hope that helps!