What key is this song in?
I wrote a blues song that's main riff is a D major chord, then each turnaround goes to G Major, C Major and then back to D Major. I'm confused because those chords tell me that the song is in G Major but the song mainly rides the D Major and all the lead guitar plays in D minor. What key is this technically in?
Depends on how deeply you want to look into it! The chords are taken from G major, as you correctly mentioned, but the key centre is D major.
There are a couple of reasons, like I said, all about how deeply you want to look into it. The short answer is:
The key is D major. This is because the chords are taken from D Mixolydian/G major and the 'home chord' where everything resolves, is D major. D minor works over the chords as a lead because it's playing some 'cool' notes against the chords.
The notes in the chords D, C and G major have these notes - D F# A; C E G; and G B D. If we make a scale out of these notes, we get a G major scale (G A B C D E F#) which is where our chords come from. But G major doesn't sound like the key centre - D major does. So let's rearrange the notes of G major, starting on D:
D E F# G A B C
Starting on the fifth note of the major scale like this gives us the Mixolydian mode, more specifically in this case: D Mixolydian. So the chords all come from D Mixolydian too because the notes are the same as G major. D Mixolydian gives us our D major 'home chord' and the chords of C and G major too - it's definitely the right fit! G major gave us our chords, but not our 'home chord'.
As a slightly more complex alternative, you can also look at the chord progression being in D major rather than G major. So the chords Dmaj to Gmaj to Cmaj would be, in roman numerals, I IV bVII. The C major chord, as we know is not diatonic to D major but we can just say that the C major is 'borrowed' from another key or mode. (In this case, it would have been 'borrowed' from D Dorian and the whole topic of borrowing chords is called Modal Interchange).
- Blues often extends these major chords to Dominant 7 chords - D7, G7, C7 which gives each chord an independence. They are all derived from the Mixolydian mode of various scales, but function as a I chord (they sound like the 'home' chord).
So, what key would you say it is in during everyday conversation - G major, D major or D Mixolydian? (Providing you had some weird conversations!) The most accurate answer would be D Mixolydian as we discovered above. In reality though, not many people are this specific as to what mode the piece is in when describing the key of a song. Therefore D major is just as appropriate as we usually base tonality of songs on just major and minor, describing where the 'home chord' is.
As for playing D minor over all the chords, this is very common in blues and the F (minor 3rd of D) could be respelled as the #9 of the chord as you already have the major 3rd (F#). this creates tension and is a really colourful/cool note.
Sorry its so in-depth it's quite a big topic! Any questions or clarification let me know and if anyone has any suggestions to make it simpler or corrections let me know!
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