The seven tones of the diatonic scale are the building blocks for creating melody and harmony. Chords in diatonic harmony are generated from the major and minor scales by constructing triads on each of the diatonic scale degrees.
Diatonic means any note or chord that is within the scale. Passing tones are many times not diatonic, but we play them, and sometimes we pass through non-diatonic chords to get to the next diatonic chord.
In the introduction of Lessons in Harmony, I introduce the diatonic scale in the key of C. There are major and secondary chords. The major chords are: I and IV, V; and secondary chords are: minor ii, iii, and vi, and one diminished chord- vii dim.
When I first began composing at the piano, I would listen and simply keep what sounded good and discard what didn’t. Having since learned to chart a song, I realized I was writing progressions! And what I thought was random unruliness was not at all. It worked because I was following rules of progression.
A chord mapping tool provides direction in what could be a chaos of notes. The chord map organized for me the idea of a progression of chords, not just random chords strung together. Rather than playing individual chords I began thinking in terms of playing a progression of chords.
Awareness of the materials that are absolutes in the selection or choices of chords to choose from makes composing easier. Go the Introduction on Lessons In Harmony for the next lesson on harmony.
*map borrowed from A Geometry of Music, Dimitri Tymoczko*