Quartal harmony consists of chords built of stacked fourth intervals rather than the more typical stacked thirds of tertian harmony.
Just like tertian harmony, however, quartal harmony can be triadic (containing only three notes), or can include extensions, creating chords with four or more notes
It can also be diatonic – constructed using notes taken from a single key or mode – or chromatic – constructed using a combination of fourth intervals not found in any single key or mode.
As with diatonic thirds used in tertian harmony, which can be either major or minor depending on the intervals of the scale or mode used to construct the chords, the fourth intervals of diatonic quartal harmony can be perfect, diminished or augmented depending on the scale or mode from which they are taken.
In the following example, a quartal chord built on the tonic of B flat major contains 2 perfect fourth intervals and an augmented fourth between the notes E flat and A, the tritone of the key.
And in this example, a quartal chord is constructed on the subdominant of the A melodic minor scale. The resulting chord contains 2 augmented fourth intervals and a diminished fourth
When constructing chromatic quartal chords, the type of fourth intervals used is entirely at the composer’s discretion. For example, here, the resulting quartal chord contains notes not found in any single diatonic key or mode
Creating Diatonic Quartal Chords
As with diatonic tertian chords, diatonic quartal chords can be formed by harmonising a scale or mode
The following chord shapes have their root notes on the A string and are built on each degree of a B flat major scale.
Because quartal chords lack the common tertian structure of much harmony there is some ambiguity in their sound. This means that each of these chords potentially has four different root notes depending on the context in which it’s used.
For example, if the B flat note of the first quartal shape is taken as the chord’s root note, it produces a B flat major 11 sound
and if the E flat note is taken as the root note, it produces an E flat major 7 sus 4 sound.
Often, however, the root note is not contained in the quartal chord shape and is instead contained in another part, perhaps played by another instrument. In these instances, the notes of the quartal chord will typically form extensions.
In this example, the first B flat major quartal chord shape is used over an F root note producing an F13 sound.
The harmonic ambiguity of quartal chords means composers typically use them either as a ‘decoration’ of tradition tertian chord progressions or as the sole basis for a piece’s harmonic palette.
Decorating Tertian Progressions:
The following I – IV – V – I progression in B flat major uses familiar seventh chord shapes
and here, the progression has been embellished using some of the quartal chords from the harmonised B flat major scale, shown earlier.
As we saw previously, because of the harmonic ambiguity of quartal harmony, this shape
can be used over both chord I and chord V; the notes of this shape produce the 9th and 13th extensions of both chords.
Quartal Harmony as a Work’s Basis:
The following excerpt is based on four different quartal chords
which substitute for two tertian chords: F7 and E flat 7.
Rather than using the 2nd and 4th quartal chord shapes as indicated, however,
the top notes of the 1st and 3rd chords are repeated while the 2nd and 4th chords are played.
The repeated top notes of the 1st and 3rd chords, double notes already present in the 2nd and 4th chords.
The combined notes of the quartal chords produce 11th and 13th extensions over both dominant seventh chords