Question: Hey Albert, I am glad to hear of your success at the TED convention.
My two-part question is one that may sound odd but "How can I be sure what chords to play on the melody note in a song?" and "Is it scientific or just creative?"
I hope you understand my question.Aundre
Albert's reply: This is an absolutely wonderful question, Aundre. Harmonizing melodies is as much a matter of logic (i.e., music theory) as it is creativity. To understand this point it's important to address your first question.
Harmonizing melodies is a lot like playing chess in that there are many rules and countless possibilities and variations thereof for any given position. Like chess, the rules are overwhelming at first and may be internalized over time, at which point they will seem largely intuitive.
It must be said that I'm speaking mostly of conventional harmony. I consider it essential to learn—and learn to apply—the rules of harmony in order to become a more complete musician and to begin to think like a composer. There's nothing forcing anyone to stick with conventional harmony, and indeed for modern composers it is desirable to achieve flexibility in one's expressive vocabulary. That said, as long as we study classical music as well as jazz and popular music, we will need to understand their vocabularies, and the more fluent we are, the better.
As for practical advice, the first requirement is that you be able to play standard chord progressions (V-I, V-i, V7-I, V7-i, IV-V-I, etc.) in all keys. This implies knowing the three most common chords: tonic (I), dominant (V) and subdominant (IV).
Next, notice that the tonic chord is composed of scale degrees 1, 3 and 5; the dominant of 5, 7 and 2; and the subdominant of 4, 6 and 1.
Together, the three most common chords cover all seven degrees of the scale. This means that it's possible to harmonize any diatonic melody (i.e., one using the scale degrees) using only the I, IV and V chords. This is a major reason why these are the most commonly used chords in all Western music, regardless of genre.
As an exercise, practice harmonizing diatonic melodies using these chords. This exercise should only be performed once you're able to play common chord progressions fluently in all keys. (It's important to realize chord progressions with proper voice leading!)
For serious students I recommend a piano theory book, Keyboard Harmony by George Wedge. The book contains many exercises for harmonizing melodies. It's now in the public domain and is freely available; I caution that it quickly becomes advanced and it would be best to work through the exercises with a music theory teacher.