Scales and chord progressions

[mbs]

Thanks to everyone who responded to my first q. I learned a great deal. Just wanted to ask a follow up.
I'm trying to understand basic composition because i found myself mindlessly playing through sheet music and not paying attention to the theory behind a piece. I know that every piece is in a certain key and i know that chords are built from keys. so for instance the 7th chord of Eb is Eb-G-Bb-C. OK. That's the extent of my theoretical knowledge. I'd like now to take the next step and understand how a composer determines which chords to use. Does a composer hear something in his head and figure out what chord it is? Are chords a way maybe of organizing his composition? Is this the purpose of chords?
Thanks a lot for the help!!

[gary]

...so for instance the 7th chord of Eb is Eb-G-Bb-C.
Oh, mbs, I'm afraid you're on the wrong track altogether. The 7th chord of Eb is D half diminished (or dmin7(b5). That is D-F-Ab-C. Or, if you mean a 7th chord built on Eb it is Eb.G-Bb-Db. Or if you mean an EbMajor seven, it's Eb-G-Bb-D. Regardless of what you mean you're wrong on all accounts. The chord you spelled is an EbMaj6 or just Eb6 chord.

mbs, none of us was born with an understanding of theory and it just takes time, so you're not the only one. However, before you start worrying about composing I suggest you get a'hold of a good theory book or have someone explain more theory to you. You won't be able to grasp composing until you have a good grasp of at least basic theory.

Regarding your question on the composing process, composing can be completely melodic, with the harmonies a consequence of melody. Or someone can take an existing harmonic progression and write a melody to it. Or a combination, where the harmonies and melodies gradually evolve together.

The short answer to your question about the purpose of harmonies is, traditionally, to give forward motion to the composition. This motion can be interrupted or even static and in post-Classical composition the harmonies can be used simply to give color.

[mbs]

gary,

i did in fact mean the 7th chord built on Eb(Eb-G-Bb-D(not C)). Can you recommend any basic theory books??

Thanks.

[gary]

Check out this online rexource. It looks pretty good...and it's free.
http://www.dolmetsch.com/theoryintro.htm

[Sassaphone]

I just bought a terrific book. It explains everything in detail and assumes nothing. It's called "Jazz Technicques" by Chase Sanborn. It also acts as a jazz instructor--that's assuming you don't already have one.