“Scale” and “Key” are not the same thing.
The terms “key” and “scale” are usually interchanged and confused.
Very few musicians realize there is a difference.
For example: You could be in the key of C, but in the scale of Ab Lydian.
Later on: I will explain how this is possible.
Scale: series of notes
Key: gravitational center that the song keeps coming back to. It is the one note you can sing throughout a whole song, without that note ever clashing anywhere with the chords in the song.
A song you write could be in a certain key throughout the whole song, yet be using different scales within that song.
One of the easiest ways to showcase that distinction between key and scale would be with modal interchange chords.
Here are a couple of examples to showcase the difference between key and scale.
Cmaj7 | Fmaj7 | G7 | Abmaj7 |
The above chord progression is in the key of C throughout all 4 chords.
The Ab chord is a modal interchange chord taken from a parallel C scale (the key of C minor), to be used in the C major scale chord progression above.
Since the Abmaj7 chord is taken from another C scale, the above chord progression never leaves the key of C.
However, you could not play a C major scale over the Abmaj7 chord.
You temporarily have to switch to a Ab Lydian scale. (Ab Lydian is relative to C minor scale)
Cmaj7 | Fmaj7 | G7 | Gm7 |
The above example is again in the key of C.
The Gm7 chord is a modal interchange chord. It is borrowed from a C minor or C Mixolydian scale scale.
The chord progression, never leaves the key of C, because the Gm7 chord is borrowed from another C scale.
However: the scale of choice over the Gm chord would be G Dorian, which is relative to C Mixolydian scale
Cmaj7 | Fmaj7 | D7 | G7 |
This example has a secondary dominant D7 resolving to G7
Again we never leave the key of C in this example.
The D7 is the (V) dominant of G. It is also one of the chords in a C Lydian scale.
While this example is entirely in the key of C, you would again have to change the scale over the D7 chord.
The scale of choice over the D7 chord is the D Mixolydian scale.
Not only are “key” and “scale” easily confused or are the words interchanged incorrectly, you could be in a different key and scale simultaneously.
The above examples show how you could be soloing with an Ab or a D scale while you’re in the key of C.