I've found a really cool chord progression that I've been playing on guitar that goes Bm-D-G-G. I'm trying to create a chorus to go with that chord progression, and I've settled that I'm playing in the key of G. It's possible that I am just naïve seeing as my theory education is kind of haphazard, but I've never really seen a iii-V-I chord progression, and I can't figure out a chord progression that would fill the chorus. Is it possible that I am searching in the wrong key?
The roots by descending fifth at
G and the holding of
G for two bars certainly sets up
G as a tonic and then you get
iii V I I.
But, there is a potential ambiguity. One way to "check" things is pick another diatonic minor chord for the starting chord, step through the changes and see if the root changes and chord qualities are diatonic. For example, label
vi, then continue the progression root
vi up a minor third to a major chord
I root descend a perfect fifth to a major chord
IV. It technically could be
vi I IV.
The tonal chords
V I IV hold their important status, because you cannot reposition them as we did above. (You can do the same with
ii V I.)
Another way to avoid ambiguity is to add a seventh to the supposed dominant. There is only one diatonic dominant seventh chord in major key. If you made
D7 then it becomes clearly
iii V7 I I.
Of course you don't need to "clarify" anything. You can leave it ambiguous. Lot's of rock/pop music uses such "ambiguous" progression deliberately. (They lend themselves to endless repetition.)
If you wanted to take a more traditional route, you could use a combination of adding the seventh, include additional dominants, and re-ordering the main chords to move the tonic around.
F#will tonicize the
Bm D G G | F#(7)...is then
i III IV IV | V(7)...with a half cadence feel
Awill tonicize the
D Bm G G | Bm Bm A7 A7is then
I vi IV IV | vi vi V7 V7
Ccould clarify the
Bm D G G | Bm C D D
Those are just examples. You do lot of other things. I think adding in the extra dominants tends to invite longer phrases and set up phrases 1st & 2nd endings possibilities.
You may not like the sound of that kind of harmony. (When I play it, it sounds like some kind of folk rock.) Even if you don't want to do that kind of thing, it does "fill in" the basic theory by demonstrating how to define the tonic.
You could simplify a few of those ideas for making a chorus or bridge. Some kind of change from tonic
G to tonic
D is one way to structure a verse/chorus. And some kind of move to the mediants
iii with a continuation to
V is a common pattern for a bridge.
Looking at it from a different perspective, you could easily be in Bm. All three chords belong there too. If you wanted, the next chord would be F♯ - the V of Bm. It could lead into a middle section.
Bm, D and G all belong to key D as well, so there's lots of options here. And bear in mind that there's absolutely no law that says you must play everthing diatonically.
You are not naive and your choice of key is a good one for this.
There are plenty of examples of this type of progression and the Bmin is a very interesting chord in this context for the following reason(s):
The iii chord is a viable substitution or extension of the I chord. from poly chord theory the I + iii makes a I maj7.
The iii chord is the relative minor substitution for the V chord.
Either way you look at it the iii can serve as its own identity harmonically or (1) as a voicing of the I maj7, or (2) as a rel minor sub for the V chord.
Above all, if it sounds good keep doing it.
A chord progression is not necessarily in any key. They key is in your head and it's defined by where you imagine the tonic to be and if the tonic is a minor or major chord. The chord progression you gave could be used in different keys, at least G and D, why not even A. You can affect the feeling of a key not only with chords but melodies. I made two different example melodies fitted over the chords, first in the key of G, then in D.
Bm D G G in a song in G
Bm D G G in a song in D
As you can see, the main melody outlines the tonic triad of the key. In the first example the melody uses the notes G, B, D heavily, especially B, over the Bm - D - G - G chords. In the second example there's a similar melody, but this time on notes D, F#, A, the notes of the D major triad. This helps in placing the whole song in a key.
The interpretation of iii-V-I in G Major is a fair one. The iii chord can serve as a stand-in for the tonic chord. It's not a progression one studies at conservatory, but certainly doesn't violate any rules -- and sounds good, rules or no.
The progression has some nice potential, too, in that you could easily shift to B minor without changing the progression. Just hold the B minor chord longer, and make the D-G chords relatively short. That transforms the progression into something that sounds like i-V/VI-VI in B minor. (V/VI means the V chord relative to the VI chord of B minor. One could call it the III chord, but V/VI is more description of its function.)
It's all diatonic in G major. All in the ballpark. What's the bass line do? Playing roots, B, D, G, it outlines the tonic triad of G major. Or the bass could go B, A, G - a scale is always strong, particularly one leading to the tonic. Don't feel you have to force it all into a 'cycle of 5ths' pattern (though there's the obvious V - I in there).
Chords for the next bit? Wide open.