Knowing the intense hatred most people have for music theory, I'm going to keep this very simple. Starting with the key of C at the top of the circle - C has no flats or sharps.Visually on the piano keyboard, there are no black notes used, and the key signature sign shows no flats or sharps.

Every step in the clockwise direction adds a sharp to the key signature (the squiggly bit at the start of music scores) and also to the major scale. So G has one sharp, and goes

G A B C D E Fsharp G (the sharp sign is like a hash key)

then D goes: D E Fsharp G A B Csharp D - and so on. (two sharps)

Now we're going anti-clockwise, adding a sharp each step. Eb is a very important key, as so many jazz standards are written in this key. Ab is very uncommon by comparison.

First, we can identify the key of any written music score. The ii V I chord progression and the V I progression are the two most prevalent chord changes in music. Just by using adjacent steps we can work these out for any key. The ii is always minor, the V is always a 7th, the one is always major.

In D: Em A7 D (shown on the diagram)

In C: Dm G7 C

The V - I would be G7 to C, E7 to A, C7 to F, etc.

Another common progression, especially in jazz standards, is the minor ii V I.

In this type of chord sequence, the first chord is m7b5, second chord is 7th, last chord is minor.

Using C minor as an example, you would have Dm7b5, G7, Cm. Blue Bossa is one standard that uses this sequence. Dance me to the End of Love by Leonard Cohen is another.

Most jazz tunes are a series of ii V I or minor ii V I chords.

Another way to use this cycle: From any point, the notes on either side give you the I,IV,V chords in any key. For instance, A has D and E adjacent to it - the three major chords in that key.

To play a blues in A, just use A7, D7, and E7.

All this material should be invaluable to songwriters. In fact, putting it into practice will really help you understand the mechanics of harmony and music.

Try playing the progression Am D7 G maj7 Cmaj7 - used in countless songs, but Autumn Leaves and I Will Survive (with Dm replacing D7) are good examples. Perfect Day is another(Lou Reed), and The Beatles You Never Give Me Your Money from the Abbey Road album

My new hub guitar and piano - easy theory might help if you are new to music theory. I find a little goes a long way - maybe combine 20 mins a day actual practice with 10 mins theory and your musical development might really take off.

The cycle of fifths is also the cycle of fourths , depending whether you are going clockwise or anti - clockwise.