I know my question looks similar to many other questions. But, please be patient with me. I'm writing this question because I can't find any appropriate answer although I have searched for it for a long time. I read some books about music theory, got some private lessons, and searched Internet, but unfortunately, I was not able to find the answer.
I've learned and realized follows through my search, but they can't completely answer my question.
- Functions of chords are basically one of tonic, dominant, and subdominant. I've learned that tonal music is a kind of a journey that starts and ends with tonic.
- Diatonic chords and secondary dominant chords towards one of them consist a pool of basic available chords in tonal music(I know there are many exceptional cases when the tonality of a song changes, etc).
- I can confine a set of plausible chords for a specific measure by comparing notes of each chord of the pool with notes of a melody on strong beats except passing tones.
- I can find plausible chord progressions by trying each chord of the pool(diatonic chords first, others afterwards) on my instrument and judging by ear.
But it's very tedious to try every (diatonic) chord in every measure of a song to accompany it even though you can guess some of them with typical chord progressions like ii-V-I. And this approach is not applicable when you need to accompany a song immediately.
I know there's no one chord for a measure that we can all agree. You can use IIm7 - IIb7 - IM7 instead of IIm7 - V7 - IM7 if you want make different sensation. That depends on individual sense. But we can say IIb7 and V7 are members of a plausible chord set for the measure at least. In the other hand, there are chords which just make no sense. I'm wondering if there is a theory to decide a set of plausible chords for a measure where the whole melody of a song is given.
For example, let's assume that there is a song of C major and its melody is as follows. Every note is a quarter note.
C C E | E E D | C C A | G G G |
C C E | E E D | G G G | G G E |
It's impossible to decide chords for a measure when only the measure is given without context. The fourth measure of the above song has only note G and it belongs to I, iii, V. But we can't say all of them are plausible because iii doesn't sound good. So I suppose a theory to decide a set of chords should explain its rule based on not only notes on strong beats of a measure but also the context of the measure (i.e. melody or chord before and after the measure). But I've never seen a theory like that. If there is a theory like that, is it possible to show how to decide a set of plausible chords of the fourth measure of the above song based on the theory?
I'm not actually interested in theory itself. What I really want is the ability to accompany a song that I know very well without chord sheets. I saw some people can do that, and I tried in various ways, but I still can't. My teachers were not able to answer my question. So I feel desperate.
If such an algorithm exists and someone knows that, there should be a paper or a book about it. I'd really appreciate if you mention one of them. If no algorithm about it has been found, it must be a subject worth studying. But I need to start with figuring out whether such an algorithm exists.
Thank you for reading my long question. My question became long because I was desperate to explain it well.