Key Centres

If you're reading from a piece of sheet music, then you should be able to tell what key you're playing in by referring to the key signature. However, if you don't have the written music in front of you, it's still possible to find the key centre (finding the key centre is another way of saying that you work out what key the piece is in.

The trick (if you can call it that) is to look at the chords used, and to determine which key(s) each chord is found in. You should be able to spot a key that applies to all the chords. This can be a bit more reliable than simply looking at the notes involved and trying to fit them to a parent key - it is not unusual to find outside notes (i.e. non-diatonic notes; notes which do not come directly from the key) in a piece of music as they can add a certain colour or tension.

Certain types of chords occur on certain degrees of the parent scale. Consider the 7th chord harmony of the major scale:

I7

iim7

iiim7

IV7

V7

vim7

iim7

viių7

Now let's look at a simple chord progression. There are only three chords used, but these are enough to be able to find the key centre. Have a look at the progression written out below, and notice that each chord is annotated to show which key it could belong to.

Dm7	G7	C7	C7
ii - C iii - Bb vi - F	V - C	IV - G I - C	IV - G I - C

Now let's take a look at the line of reasoning involved in finding the key centre:

1. Dm7 could belong to any one of three keys. It could be the ii chord of C major, the iii chord of Bb major or the vi chord of F major.
2. G7 is a V7 chord (dominant 7th type) which only appears once in any given major key, always as a V chord. Therefore, there is only one possibility here: the V chord of C major.
3. The C7 chord could be the IV chord of G major or the I chord of C major
4. Now the progression can be examined - we need to look for a parent key which is common to all the chords.

In the case of this example, the key of C major is evident:

Dm7	G7	C7	C7
ii - C iii - Bb vi - F	V - C	IV - G I - C	IV - G I - C

Now let's have a look at a more complex example. Here there is a key change (modulation) part way through, moving from Ab major to C major:

Fm7	Bbm7	Eb7	Ab7	Db7	Dm7	G7	C7
vim7	iim7	V7	I7	IV7	iim7	V7	I7
Ab major					C major

In detail:

Fm7 = ii of Eb iii of Db vi of Ab	Bbm7 = ii of Ab iii of Gb vi of Db	Eb7 = V of Ab
Ab7 = I of Ab IV of Eb	Db7 = I of Db IV of Ab
Dm7 = ii of C iii of Bb vi of F	G7 = V of C	C7 = I of C IV of G

For reference, you might find charts of parent keys for major, minor and diminished triads. Also for reference purposes, there are charts of parent keys for major 7th, minor 7th, dominant 7th and half diminished chords.

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