Quartal voicings refer to chords that consist of stacked 4th intervals (perfect or augmented). Compared to tertiary harmony (voicings based on 3rds), quartal harmony has a more ambiguous and spacious sound.
The number of voices in a quartal voicing can be between 3 (played as left hand voicings) and 6 (full voicing with 3 voices in each hand). To begin let’s take a look at 3 note left hand quartal voicings and how they may be applied to major scale harmony. There are 7 possible voicings for each key, built by stacking 4ths below each note of the major scale. It is useful to think in terms of stacking 4ths below instead of above, before the top voice for each voicing is the most dominant voice and can often be a part of the melody or countermelody. Thus for the key of C, we have:
Let’s try to apply these voicings to the 4 most common chords in major scale harmony, namely I-maj7 (Ionian mode), II-m7 (Dorian mode), IV-maj7#11 (Lydian mode) and V-7 (Mixolydian mode or dominant):
In the figure above, some voicings are labeled as “avoid”, because of the presence of “avoid” notes (the 11ths of I-maj7 and V-7 chords create dissonant minor 9th intervals with the 3rds).
Note that most of these voicings do not contain all of the chord quality defining chord tones. For example, for the Cmaj7 chord, only the B-E-A voicing contains both the 3rd and the 7th necessary to define the major 7th sound. This is the reason why quartal harmony can be ambiguous, because one voicing could be applied to multiple different chords. For example, the E-A-D voicing works not only for Cmaj7, but also C6, C7, Fmaj7, Bbmaj7#11 and more:
Let’s add one more note to the voicings, and see how they apply to the commonly encountered chords in the key of C:
With more notes added, less voicings are usable for major 7th and 7th chords due to avoid notes. The minor 7th and the major 7th #11 chords, on the other hand, do not have avoid notes. However note that not all the voicings contain the color defining chord tone for the chord. For instance, only 4 out of 7th voicings contain the color defining #11th for the major 7th #11 chord.
Adding one more note we have five note fourth chords:
And finally, six note fourth chords:
Notice that with 6 notes, there are practically no usable voicing for the major 7th chord: 6 of these voicings contain the avoid note, and 1 contains a dissonant minor 9th interval. For the 7th chord, only 1 voicing is usable while the other 6 all contain the avoid note.
While minor 7th and the major 7th #11 chords have no avoid notes and all 7 voicings are technically usable, some may better be avoided because with 6 notes in the voicings we now have occurrences of dissonant minor 9th intervals. However, these can be also used to create tension especially when playing a modal tune with relatively unchanging harmony.
It is also possible to get around the issue with the avoid notes in major 7th and 7th chords by slightly modifying the chord symbols so they become major 7th #11 and sus 7th chords respectively. If we play the major 7th chord with a #11th, we are effectively re-harmonizing it to the lydian mode of a different key. And by playing the 7th chord as a sus 7th chord, we are effectively saying the 11th is no longer an avoid note and we can use all 7 voicings freely:
To become familiar with the shapes of fourth voicings, one should go through all 12 keys quartalising each mode of the major scale.
The same chord building technique can be applied to melodic minor harmony, in which there are also diminished 4th’s in addition to perfect and augmented 4th’s:
C melodic minor:
C# melodic minor:
D melodic minor:
Eb melodic minor:
E melodic minor:
F melodic minor:
F# melodic minor:
G melodic minor:
G# melodic minor:
A melodic minor:
Bb melodic minor:
B melodic minor:
Finally, the same chord building technique can be theoretically applied to diminished harmony. By quartalising each note along the diminished scale, chord sequences with very unusual sounds arise. There are probably not many chances to apply these almost eerie sounds, but it is good to know that these possibilities exist.
C, Eb, F# and A diminished:
Db, E, G and Bb diminished:
D, F, Ab and B diminished: