cool, at least there's a thread about it now, and as it has been the most important aspect of music driving musical revolutions over tha past 600 years, seems strange that some songwriters give it little serious thought other than this chord sounds nice, or that chord sounds nice..
As tone says, harmonic theory is not concerned with vocal harmonies (although it is related). How a vocal harmony moves through the chords is known as voice leading. Also, as tone says, extending basic triads beyonf the normal 1,3,5 can really enrich not only the sound of the music but also how you write. Try singing on a B with a C major chord (Cmaj7th).
harmonic theory is concerned with chords, and how they progress, and underpins melody and structure.
Yes, you can do all this by ear, as songwriters have done for decades, but you will be suprised how many of the 'master' songwriters have a working knowledge of this. Some styles such as Jazz rely on complex harmonic progressions.
Ramshackles, what would help you would be 'pivot chords', i.e. a sequence of chords that belong to the original key and the target key. Also an understabding of ii V I progressions can help create interesting modulations. I had a complex one worked out for a bridge I'm working on in that song I posted, but when hearing it back we've gone with a three chord trick, sometimes the style dictates the harmonic progressions required.
And tone, yes, once you get those diminished chords under your belt it opens up a whole new harmonic universe, and ways of modulations, coupled with ii V I progressions, the sky is the limit!
Chainsaw, for some styles, three chords is all it takes!
anyway, just thought I'd open it up, be nice to see discussions on chord progressions, key changes, etc
CHris
www.chrislong.me.uk@ChrisLongCOMP