As far as harmony goes, this set of chords will be one of the most important things you learn. These are the diatonic chords of the Major Scale. Diatonic means coming from or derived from a scale or key. So, for example, if our key is C major, then the notes of the key are C, D, E, F, G, A and B. While in the key of C, playing or using any of those seven notes is considered diatonic. Notes outside of the key, such as C# or Gb, would be called chromatic.
Many songs are purely diatonic. That is, all of the notes of the song stay within, or come from, the seven notes of the key. A chord which is diatonic is simply a chord built from notes of the key.
In the key of C again (C, D, E, F, G, A and B), the chord C major (C, E, G) would be diatonic to the key of C because its 3 notes are part of the C major scale.
The C minor chord (C, Eb, G), however, would not be diatonic because the third of the chord (Eb) is not in the key of C major. We’ll get into non-diatonic chords within chord progressions as you progress through Guitar Super System.