For instance, when playing in the key of C major, you can incorporate chords from the parallel minor scale – C minor. The parallel minor scale should not be confused with the relative minor scale of C, which is A minor. While a relative minor scale contains the exact same notes as its parent major scale, a parallel minor scale contains different notes. This is why borrowed chords from a parallel minor scale are considered non-diatonic.
Another possibility of incorporating non-diatonic chords is by using secondary dominants. In essence, a secondary dominant chord is the dominant chord of a dominant chord. In a progression, we can use these chords to resolve back to the tonic.
In Theory Lesson #6 we learnt about the harmonic function of chords. Thus, we know that the fifth (‘V’) chord in a major key has dominant function. A dominant chord is a triad with an added minor 7th (ten semitones from the root note). The dominant chord (‘V’) of F is C. The dominant chord of C is G. Therefore, the secondary dominant (‘V7/V’) of F is G7. We can use this secondary dominant chord to resolve from F (‘IV’) to the tonic, C (‘I’) by playing ‘F – G7 – C’. This is a great way to resolve harmonic dissonance.
Feel free to ask any questions about the use of non-diatonic chords in the comments below. And if you are ready to deepen your understanding of music theory – visit our shop for the Songwriter's Pack, which contains everything to get you started with learning music theory.