DIATONIC CHORDS USAGE

Diatonic chords are the most basic chords that are used to make harmonic support. When we have a melody written in a key and we want to make harmonic support for this melody, a good starting point is to use the diatonic chords from the same key. Any diatonic chord that contains the note name of a note from the melody can be used as the harmonic support for this note. Such a diatonic chord sounds natural because it firmly belongs to the key and is in a harmonic relationship with the note. The next figure shows an example of a passage containing only diatonic chords. It is taken from the beginning of Chopin’s Prelude No. 9 in E major.

Figure: A passage containing only diatonic chords from Chopin’s Prelude No. 9 in E major

All the harmony annotations in the figure can be found in chapters Diatonic triad chords in major keys and Diatonic seventh chords in major keys. Thus, all the chords in the figure are diatonic chords in a major key. Since the key is E major, they are diatonic chords in the E major key. The next figure extracts the melody (notes with the highest pitch at every moment of time) and also shows the chords. We see that each note from the melody is accompanied with the chord that contains the note name of the note. For instance, the first note of the melody is B3 and it is accompanied with the E major triad chord which contains the note name B. The only exception is one nonchord tone (suspension) which appears in measure 3. 

Figure: The melody and chords from Chopin’s Prelude No. 9 in E major

However, a long passage that contains only diatonic chords tends to sound somewhat dull. This is why composers also use other types of chords to make chord progressions more exciting. These other types of chords will be explained later. For instance, if we return to the harmony analysis of Chopin’s Prelude No. 7 in A major, we see that this short composition is completely made of diatonic triad chords, except for a single chord (F# dominant seventh chord) which indeed appears in the most exciting moment of the composition.