It's a truism, which also happens to be true, that Western music is characterized by polyphony, unlike classical Arabic music, for example. In Western music, since the Middle Ages, when people started singing in counterpoint in church services, melodies have been accompanied by other melodies, sung and played at the same time. Recently, with a friend of mine, another flautist, I have been reading through duets by Stamitz, a German baroque composer, a case in point. All there is to this music are the two melodic voices, which both answer to each other and blend together. Occasionally one or the other voices arpeggiates chords under the melody played by the other voice. But are they chords?
The question in my mind is: When did Western musical theorists and practicing musicians invent the concept of chords? When one studies Western music and classical harmony, chords are axiomatic. We learn chord progressions: one chord leads to another, from an initial statement of the key in which they piece is written, on and on, until the progression resolves by returning to the original key. When you know something about harmony, as you play the melodies in pieces like the Stamitz duets, you notice when he moves you from one key to another, and from one place to another in the chord progression underlying the piece. But medieval musicians didn't think in terms of chords. They thought in terms of modes, in which complex, interlocking melodies were performed. Later on baroque and classical composers added the idea of chords and chord progressions to their works: functional harmony.
Since we Westerners are accustomed to hearing to music based on functional harmony, we listen for it, even when it's not there. In the second half of the nineteenth century, composers like Wagner (about whom I don't know that much) and Debussy (about whom I know more) broke free of the functional harmony that underlay classical music, but their music still has a texture that the listener can relate to. However, theorists like Schoenberg sought to free music of the idea of the tonic, the idea that a certain note was the most important one, the center from which one departs and to which one returns. His music still makes us feel uncomfortable.
Jazz was originally a harmonic genre of music, not surprisingly, because a lot of it was played by ear, and our ears are accustomed to harmony. But increasingly jazz musicians have been straining against the restrictions of traditional harmony. Recently I've watched some videos that explain new conceptions of jazz harmony, the use of non-traditional scales and strange stacks of notes that they insist on giving names to, as if they were chords in the old-fashioned sense.
The basis of functional harmony is voice-leading. To put it very simply, a G7 chord, GBDF, resolves to a C major chord because the G stays the same, the B moves up to a C, and the D moves up to an E. Jazz musicians discovered that the two important notes in the G7 chord are the B and the F, known as a tritone, with a very unstable sound. They also noticed that the Db7 chord, Db-F-Ab, and Cb, contains the same tritone: F and Cb (=B). So you can substitute a Db7 for a G7, which is odd, because the Gb scale, to which the Db7 belongs, is quite distant from the key of C major. By using a Db7 instead of a G7, you make it possible to play notes that don't belong to the key of C major. This is just one of many innovative aspects of jazz harmony, and may also explain why many conservative listeners are not fond of modern jazz.
My takeaway is different. I find myself wondering why we insist on labeling groups of notes as chords (i.e., G major 7 sharp nine, sharp eleven), unless it's for the convenience of not spelling them out (G-B-D-F#-A#-C#) which is not to be sneezed at.
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