How to identify a key given a certain chord progression

Question

At the age of 53 I'm new to music theory and in particular to chord progressions. While playing around with the piano, I've noticed that I like the sound of this progression

Cm Gm AM Dm

I have tried transposing it to other keys and sometimes switch to them for fun.

Am I playing in Cm? If so, would it be correct to use this notation to describe the progression?

i v VI ii

What I don't understand is what key I am playing in. To complicate matters sometimes I change the last chord in the progression to a major (so Dm becomes DM).

By identifying the key then I feel I should be able to improvise with my right hand.

(My confusion: if it is Cm, then the sharps and flats are different, but it almost "feels" that it's in Dm because it seems to resolve to Dm. But again the C blues scale seems to work on the right hand...!)

Thanks

Answer 1

There is no theory to tell you which key you're in. Different people can hear the same chord progression in different ways. Of course there are very clear cases of chord progressions, but yours is not one of them. The notes of these four chords are not from only one scale. This doesn't mean that there is no key, it just means that things could get ambiguous.

You feel that at the end it resolves to Dm, but that's simply because you have a (secondary) dominant right before it (A is the dominant of Dm). When I play the progression I hear it in G minor. I hear Cm as a iv chord, A as a secondary dominant, and Dm (or D) as the v (V) in G minor. Another hint that it might be in G minor is that you sometimes switch to D major instead of D minor. The latter is a characteristic of minor, namely that its V (or v) chord can be either major or minor (because there are three minor scales, and a piece in minor can make use of all of them).

But again, nobody can tell you what you hear as the root chord in this progression. Since there is no single scale really fitting all four chords, even for improvising it's not important to know the key. It's much more important to know the chords and the chord tones. You could start by a chord tone solo (i.e. a melody only consisting of chord tones), and then you can become a bit more experimental and try other notes connecting the chord tones.

One thing you might want to try is G natural minor (aeolian) over Cm and Gm, and D harmonic minor over A and Dm. If you use D major instead you could switch to G harmonic minor over D major.

Answer 2

'The key' is whichever key you choose to play these chords in. You've labeled it as c in this case because you identified the chord on c as i.

The Roman numeral version is an abstraction of the chord progression that can be played in any key. You can choose to start that chord progression on any note (in this example, you chose a C), and you will have the same effect in terms of how the intervals are related to each other. So you could start on a G (still i), or a G# (still i), or an Ab (still i, and yes, those last two are the same). What those Roman numerals are telling you is how to form the chords based on whatever note you are starting on--if C is i, then you are in the key of c as that is your tonic.

Basically, you're conflating the pitch names (C, G, F, D) with the Roman numeral chord symbols (i, iv, v, ii), which are two different systems. A C is always a C on the piano. A i depends on where you choose to place it; in the key of c it's a i; in the key of g, it's also i.

There are many more rules in Western music about which chords can go where and when, but that is getting into another topic. This is more a question of the style you are playing than what you are talking about in this example.