Most of the music we now listen to, and have listened to since the modern era began in the West, has one thing in common tonality. This music is tonal in nature from classical to romantic, from rock to pop, from blues to jazz to country, and even including rap, hip hop, and R & B. We have seen what it means to be tonal: a particular tone (or sometimes set of tones) acts as the weighty center of the music. All of the chords and scales and notes that are played or written in that piece take their harmonic function from that center, and all of them can be seen as either moving away from or moving toward that weight. This is not, however, the only way for harmony to be organized. In modal music, the center of gravity that was a single note (a tonal center) is now replaced with a network of notes a mode or scale. There are then, generally, seven notes, all acting as the weighty center (taken together) of the music. There is a key, and in some respects it can be said to center (on a scale), but there is no one note around which the harmony of the piece congeals. The notes of the scale used as the harmonic foundation are all treated equally one is not seen as the tonic, with the others serving to build or release tension vis a vis that tonic. Now there are in some sense seven tonics, and the relationships between them emerge out of the specific intervals the chords and the melodies that the composer or improviser creates within that scale (or outside of it). The song is said to have a key, but the key is written as something like “A Dorian” rather than “A minor,” with “A” in the modal sense only being there to indicate the scale being played and not necessarily the tonal root of the piece.
This is the basic idea of modal harmony, and it is as old as pre-modern European music (prior to the Baroque period and going back as far as Greek music and Gregorian chants) and much of the music that has historically come out of Asia and Africa. Modal harmony was revitalized at the turn of the 20th Century by French composers such as Debussy and Ravel, and in jazz it was championed in the mid-to-late 50s by Miles Davis, John Coltrane, and Bill Evans (among others). Since the invention of modal jazz, modal harmony has been a ubiquitous part of rock, fusion, country, free and avant-garde music, and it has always been a feature of the blues. Few things in music are as powerful or as adaptable is modal harmony.
We have already mentioned modal substitutions. These are part of the heart of modal music. When there is a chord progression, the traditional method for harmonic substitution is to alter, add to, and invert the existing chords, ensuing in new chords that share important chord tones with the old ones. In this way, we tend to are sure to end up with a set of changes that functions in the same way, or in much the same way, as the old one functions, that is, tonally. That way of substituting chords is based on the function of a chord within tonal harmony (vis a vis a tonal center). The idea is that the new chord has a similar function as the old one. There is, however, another way of going about substituting a chord. If, rather than seeing each chord as having a particular function with respect to a tonic, and each note of each chord having a particular function with respect to a root, we see instead all of the notes as being inside (or outside) of some scale or series of scales, then the game is different. We can now, since the chords are simply cuttings of a larger scale or mode, replace that chord with any other chord that is also a cutting from that same scale or mode. If we see an Am7 chord, and we assume for the time being that it is part of a G Dorian scale (or A Phrygian) then we can replace it with any chord also contained in that G Dorian scale (such as, for instance, a Gm13sus4 chord.