Diatonic Scales and Chords
The music theory term “diatonic” is usually intended to mean “of the scale”. For example, if you’re playing in the key of C major and you’re using C major scale notes, then you would say that the notes you’re using are diatonic. If you play notes that are not in the parent major scale, such as chromatic passing tones, then you would say that those notes are not diatonic.
The same applies to chords. Diatonic chords are chords that stem directly from the parent major scale. In the key of C the diatonic chords are : C, Dm, Em, F, G, Am and Bmb5. Any extensions added to these chords (such as Csus4, Dm9, Fmaj7, G7, etc.) would still be considered diatonic as long as the extensions added are notes from the scale. If you built a chord that uses notes not found in the parent major scale, then you would say that the chord is not diatonic.
Many songs are diatonic in that all the notes and chords used in them stem directly from one parent major scale. For example, the song “La Bamba” by Los Lobos is in the key of C and every note and chord from every voice and instrument uses strictly C major scale notes. So you would say that the chords, melodies and solos are all diatonic. “Like a Rolling Stone” by Bob Dylan is also in the key of C and completely diatonic. I’m using the key of C as my example here, but the same holds true for any key.