Reharmonisation - A brief snippet of what I've done so far.

Below is a brief snippet of a simple reharmonized chord sequence that I recorded using Logic Pro.

The first is the final line from the hymn "Amazing Grace".

The first play through contains the chord sequence "Bm G A7 D".

The reharmonized playthrough contains the chords "Bbdim Bm7 E7 C9 D".

These chords work very well with the melody.  The melody note during the second chord is an F# - the major 7th of the G chord and the 9th of the E chord in the reharmonized sequence.  The melody note during the A7 chord is E, the fifth.  In the reharmonized sequence however, the melody note is the third of th chord (C9).  Generally speaking, the third is a stronger note than the fifth, as its one of the two guide tones (3rd and 7th) of the chord.

So how do the reharmonized chords here relate to the original chords?

The Bb diminished is added in because adding the diminished chord a semi tone below before a minor chord is a well known and often played way of spicing up a chord sequence. This could give us a rule straight away, but I'll talk about that in another post. 

G into E7 is a strange shift, though. How does that relate? Well the fifth of G is D, which is the 7th in the E chord.  The fifth of E is also the third in G, so that remains.  We have two notes that remain the same.  The biggest difference however is the G# in the E chord - this has raised the root of the G chord by a semitone, giving us our tension.  

What about the C9? Why was that put in place of the A7? Well he 5th and 7th of the C chord (G and Bb) are exactly one semi tone above the resolving notes of the following ad chord (F# and A). As they drop down to the D, the tension they create is instantly released.  Why not just make the chord C7 then? Why does the 9th work so well?  The 9th of the chord is D - the root note of the chord that follows. By adding the d, there's a melodic and harmonic constant throughout the two chords, leading to a smoother resolution. 

When I originally chose the C9, it wasn't because of anything id heard played elsewhere orhad read - I was toying around with different chords and accidentally put in a C9.  It worked, but until i analysed it, I didn't know why.

The C9 works so well because of the chord that follows it.  This has given me another option to think about when reharmonizng - we can choose a chord to reharmonize based on the chord it's substituting, the melody note, the chord before it or the chord after it. Already I'm beginning to think about new ways of selecting chords.

While this was only a brief example, it's already given me possibilities for future rules.  Very soon I'll be posting a couple of mock rules for the rule set, which will be in some way related to this.

J