(6.12)  Key Centres

Throughout this tutorial, we have seen many examples of how the related chords of a scale are compatible with each other, and tend to appear together in chord progressions.

However this is really only half the story.  Consider the following chord progression, which is interesting for two reasons.

Firstly, it works - it creates a solid harmony that flows strongly from one chord to the next.

Secondly, no scale has all of these as related chords.  Between them, the chords above contain the nine notes C-C♯-D-E-F-G-G♯-A-B, which is two more notes than any Major scale.

What's going on here?  Perhaps you are beginning to guess the answer - this piece of music is based on more than one scale.

We tend to think of a piece of music being in a particular key, which provides a framework for the melody and chords of the music.  Many songs work exactly like this.

However, it is also very common for songs to move, after every few chords, from one transition key to another.  They will usually finish by returning to the original key.  These transition keys are called key centres.

In this way, there is actually a two-tier structure at work in music.  A progression of related chords creates a changing harmony within a key centre, while the key centre itself can give way to a different key centre with a new set of related chords.

The use of changing key centres varies greatly with different styles of music.  Simpler styles such as folk, country and pop will often stay in single key for a whole song.

However the richer and more complex harmonies in jazz almost invariably involve the frequent changing of key centres.

To put this all in perspective, let's take another look at the chord progression above, this time showing the changing key centres.  We find that three key centres are involved here - C Major, A Harmonic Minor and D Harmonic Minor.

The actual chords can now be translated into roman numeral notation, to show more clearly the harmonic role they are playing within each key centre.  For example, E7 is the V7 chord and Am is the Im chord within the A Harmonic Minor scale.

This notation allows us to 'decode' how the chord sequence works.  Essentially it is based on a repeated movement from the (dominant) V chord to the (major or minor) I chord in each key centre.

This V-I harmonic movement is very powerful, and it appears regularly in all types of music.

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