diatonic harmony
theorydy-uh-TON-ik HAR-muh-nee
Harmony built exclusively from the notes of the prevailing major or minor scale.
Diatonic harmony uses only chords constructed from the seven notes of the current key. In C major, the diatonic triads are C, Dm, Em, F, G, Am, and B°. These seven chords and their extensions form the harmonic vocabulary of most tonal music.
Understanding diatonic harmony is the first step in music theory — before exploring chromaticism, borrowed chords, or modulation, a student must master the relationships between chords within a single key. The most common progressions in pop music (I-V-vi-IV, I-iv-V, ii-V-I) are all diatonic.
The I-V-vi-IV chord progression is so universal in pop music that it has been nicknamed the Axis of Awesome progression — after the comedy group that demonstrated it underlies dozens of hit songs.