This article was originally published in Chapter 2 of Jazz Composition: Theory and Practice by Ted Pease.
The modes have provided fertile musical soil for jazz composers since the late 1950s. In effect, modes can be viewed as displacements of the major scale.
Diatonic 7th chords for each of the modes can be derived, as shown below. Each mode contains a so-called characteristic note that helps to distinguish it from major and minor, and from the other modes.
Most textbook explanations of modal harmony warn you to beware of the diatonic tritone in each mode, lest it pull you into the relative major key. This warning is valuable, but it can be somewhat limiting, especially in jazz composition. Since the characteristic note of each mode is also a note in the diatonic tritone of that mode, it stands to reason that the tritone may actually have a role in helping to establish the sound of that mode. Play the examples below on the piano and you will see and hear that these voicings, as simple as they are, are potential I chords in D Dorian, E Phrygian, F Lydian, G Mixolydian, A Aeolian, and B Locrian respectively. Note that the respective tritones are given strong support from each modal tonic in the bass clef. This helps anchor the tritone and ensures modal rather than tonal orientation.
The problem with the tritone in modal harmony is not so much the interval itself but the placement of that interval in a voicing in 3rds. Voicings in 3rds (triads and 7th chords) are so identified with the major and minor modes that their very use promotes tonal rather than modal identity. Bill Evans and Miles Davis must have understood this instinctively at the Kind of Blue recording sessions, because Bill Evans makes extensive use of voicings in 4ths throughout, especially on “So What.” Voicings in 4rths have a more ambiguous quality than voicings in 3rds. A quartal “triad” (three-note voicing in perfect 4ths) doesn’t sound major, minor, augmented, or diminished. In fact, any of the three notes in such a voicing might be the “root” of the chord! This ambiguity has intrigued jazz musicians for more than 40 years and is at the heart of the use of these voicings by players and composer/arrangers who hope to sound “modern.”
It helps to examine the diatonic chords of each mode using three-part voicings in 4ths.
Assigning Roman numerals to these voicings is not particularly helpful because there is no tonic, subdominant, or dominant quality inherent in any of them without a note in the bass. If the modal tonic is added in the bass, most of the voicings will sound “tonic,” while one or two voicings may sound vaguely “non-tonic” or like an approach chord. Then, if a note other than the modal tonic is used in the bass, all of these voicings will sound non-tonic.
The most important factor in establishing modal orientation is the frequent use of the modal tonic in the bass. This is essential because the tritone is always lurking and threatening to pull you into the relative major mode. As long as the modal tonic is used persistently in the bass voice, all of the diatonic voicings in fourths from the mode can be used above it in virtually any order to provide harmonic fluidity. The use of other notes from the mode in the bass will suggest non-tonic chords that can be used in cadencing to a modal tonic chord.
*Another nice thing about using modal voicings in 4ths is that each will contain at least one tension of the mode (9,♭9, 11, #11, 13,♭13).
Inverted Voicings in 4ths
Voicings in 4ths and their inversions have been used extensively in modal situations by jazz pianists, composers, and arrangers since the early 1960s. A three-part voicing in 4ths can be inverted by shifting the bottom note up an octave twice in succession. This results in two new positions of the voicing, which contain the same three notes but in a different intervallic order. Instead of two adjacent 4ths, the first inversion contains a 4th on the bottom and a second on top. The second inversion contains a second on the bottom and a 4th on top. (A voicing in 3rds has been avoided once again!) The chart on the next page demonstrates inversions of voicings in fourths in D Dorian, E Phrygian, and G Mixolydian.