Deciding the Key of a Song

Question by David
(SC, USA)

First off, love the website. I have a question about determining the key of a song.

For instance lets say a chord progression goes G, C, D. Couldn't this chord progression occur in the key of G as well as the key of C?

How do you decide which key you should be soloing in?

I know that if you strictly follow the rules then if this was in the key of C major then the D chord would be D minor (where as if it was in G major it would be D major). But I also saw on the site how the 2nd chord in the progression can be changed to major or minor. Could you solo in either key? How do you decide?

Thanks for all the help and this site truly is a great resource.

Key as the Musical Center of Gravity

David, firstly let me direct you to an article I wrote on determining the key of a song.

In summary, key is nothing more than what I would call a "musical center of gravity" - a chord or note that gives us a strong feeling of resolution or "arrival" in a song.

It's common (although not a stone-set rule) to start and/or end on the chord that establishes the key center. Most songs do this.

Let's take your example of the chord progression Gmaj / Cmaj / Dmaj.

If you play through these chords, there's an undeniably strong pull towards Gmaj as the "home" chord, facilitated by the other chords. If you finished the song on Gmaj you get a sense of completion, that you've arrived home, more so than with the other chords which, if you ended on them, would leave you hanging slightly.

Key can be quite subjective when progressions get more complex...

But most musicians would agree that G major is the key of the progression above, and therefore our natural scale choices would be G major and G major pentatonic.

Now, you raised a challenge to this notion by asking "what if the key was C major?"

That's certainly possible, as music doesn't have rules as such, it just has conventions (what musicians typically call "natural" harmony or melody) and we can always venture outside those conventions.

If you started the progression on G major, most would have a hard time hearing C major as the tonic (key center).

Even if you started on C major, most ears would still hear G major as the tonic, because the movement between Cmaj and Dmaj, and then to Gmaj, creates what is known as a 4-5-1 (IV V I) cadence - Cmaj being 4, Dmaj being 5 and Gmaj being 1 - a very strong key-affirming sequence.

So even if we started on the Cmaj chord, it would still sound natural to use G major as our parent scale.

Therefore, the chord you start on doesn't always determine the key. Sometimes the movement between the proceeding chords, and the gravitational pull they create, determines the key more so than which chord you start on.

Now, as you rightly pointed out, if Dmaj was instead D minor, that would change the pull of the sequence towards C major. Why? Because D minor is the natural ii (2) chord of the harmonized C major scale.

It's interesting how simply changing the D chord to minor from the original progression alters the effect the Gmaj chord has in relation to C major. It sets G major up as the V (5) chord, which in turn establishes C major as our new tonic/home.

The Dominant V Trick

But let's say we wanted to keep the D chord as MAJOR in our C major key.

There's a simple "trick" we can use here. All we do is change the Gmaj chord to a G7. That's short for "G dominant 7th" as you probably know.

Listen to how using G7 destablizes the G chord and creates a pull towards C major as the tonic...

Why does this work? Because the movement of G7 to Cmaj is a natural harmony found in the C major scale. This movement is known as an "authentic cadence" (5-1 or V-I) - a very strong key affirming movement.

By emphasising that dominant 7th in G major, a tension is created on G major that begs to be resolved to its relative tonic.

It's no coincidence that the V chord is called the "dominant".

So if ever you want to establish a new key, the easiest way to do this is to use a V7 chord. In C major that's G7. In E major it's B7. In G major it's D7 etc.

In this example, Gmaj, which sounded like our natural home before we changed it, becomes the V7 chord of C major.

The Mixolydian Trick

The above G7 trick will allow you to use C major as your soloing scale over Cmaj and Gmaj/G7.

But as the 2 chord is now major (Dmaj), it won't fit into the natural C major scale. D minor would, but D major contains a note that is not part of the C major scale (F#). We therefore have to treat this chord as an "outside" or non-diatonic chord.

So how do we know which scale to use over this rebellious chord?

Whenever the 2 chord is expressed as a major or dominant 7th chord, we can use Mixolydian over that chord. There's a reason why this works that I won't bore you with right now!

So over Dmaj, we can switch from C major to D Mixolydian. Mixolydian is exactly the same as the major scale, but with the 7th degree lowered by a fret/semitone.

We could also use D major pentatonic over D major, or a mixture of pentatonic and Mixolydian.

When the chord changes to Gmaj, we go back to the parent scale of the key - C major.

Here's a really simple example of these scale changes in action...



Click to hear

Use this backing track (Cmaj / Dmaj / G7) to try out your own ideas.

I hope this has helped! If you need any clarification, please feel free to use the comments link below. Cheers.