Modal jazz is jazz that uses musical modes rather than chord progressions as a harmonic framework.
An understanding of modal jazz requires knowledge of musical modes. In bebop as well as in hard bop, musicians use chords to provide the background for solos. A song starts out with a theme that introduces the chords for the solos. These chords repeat throughout the whole song, while the soloists play new, improvised themes over the repeated chord progression. By the 1950s, improvising over chords had become such a dominant part of jazz, that sidemen at recording dates were sometimes given nothing more than a list of chords to play from.
Towards the end of the 1950s, spurred by the experiments of composer and bandleader George Russell, musicians began using a modal approach. They chose not to write their pieces using conventional chord changes, but instead using modal scales. As a consequence of this decision, the bassist, for instance, does not have to 'walk' from one important chord tone to that of another in order to make each chord change sound, as is the case in conventional bebop or hard bop compositions; rather, he or she is free to improvise bass lines within a specific mode. In modal jazz, bass lines are often constructed in four or eight bar phrases with an emphasis of the root or the fifth degree on beat one of such phrases. Similarly, the comping instrument is not confined to play the standard chord voicings of the bop lexicon, but rather can play chord voicings based upon differing pitch combinations from the parent mode.
The way soloists created solos changed dramatically with the advent of modal jazz. In bebop, a soloist typically constructs solos to fit within a particular set of chord changes. In modal jazz, with its lack of conventional bop chord changes, the soloist can create interest by exploring the particular mode in rhythmically and melodically varied ways. Modal jazz is, in a sense, a return to melody.
It is possible for the bassist and the pianist to move to notes within the mode that are dissonant with the prime (tonic) chord of that mode. For example: within the C ionian mode, the notes of the scale are CDEFGAB, with C being the root note. Other non-diatonic notes, such as the note B♭, are dissonant within the C ionian mode, so that they are less used in non-modal jazz songs when playing the chord C. In a modal song, these other notes may be freely used as long as the overall sound of C ionian is entrenched within the listener's mind. This allows for greater harmonic flexibility and some very interesting harmonic possibilities.
Among the significant compositions of modal jazz were "So What" by Miles Davis and "Impressions" by John Coltrane. "So What" and "Impressions" follow the same AABA song form and were in D Dorian for the A sections and modulated a half step up to E-flat Dorian for the B section. The Dorian mode is the natural minor scale with a raised sixth.
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