This section discusses the construction of chords and their applications. Effective songs can use only 3 or 4 chords to express a variety of moods. The predominant chord qualities in jazz are similar to those used in the Impressionistic period by composers such as Debussy and Ravel. Major 7ths, 9ths, 11ths and 6ths; minor 7ths, 9ths and 11ths are common to both styles. Jazz introduces augmented and diminished 5ths, 9ths, 11ths and 13ths; quartal and quintal harmonies as well as unresolved chords like the major/minor. The farther away you want to go from the tonic, the more dissonance you introduce.
Sparse voicing, 2- , 3- and 4-note chords say a lot with a little. For example, a 3-note inversion of a C minor 9th is effectively voiced as D- E flat-G. The tonic note is not used, yet the chord struck alone implies the C minor tonality the strongest. Because there is no tonic in the chord, it retains a mysterious quality. The interval of a minor 2nd, major 2nd or a major 7th played by itself sounds atonal, abrasive or childish. When combined in the context of only one more note, these dissonant intervals combine to create consonance. Car horns are commonly a major 2nd. In the Beatles' 'Baby You Can Drive My Car', the vocal riff 'beep beep - beep beep yeah" is a major 2nd. Try singing it with a friend sometime.
Quartal harmonies are unresolved as they normally lack the third. Quintal harmonies like Gregorian chant are very open and can imply a variety of tonics to the listener. Hold the sustain pedal down on the piano and stack 5ths ad infinitum from any tonic and you'll recognize the often used harmony. Eyewitness News themes routinely abuse the +9 chord.
Ultimately, there are no rules. You can create any type of chord and use it how you want. Joe Pass favors an interesting method for guitar players. Let's say you have a C major 7th voiced in second position on the third fret: C-G-B-E, closed voicing or closed hand. Simply move any finger one fret up or down, 1/2 step, and listen to the new chord. Try moving the G down to a Gflat or F#. Very nice chord. Drop the E down to an Eflat. Not bad. Move the C up to a C# and you've got a half-diminished chord. Guitar players can use the low E or A string as a pedal tone and harmonize against the fundamental or an overtone. Use of a triad or chord as a pedal tone against new harmonies produces polychords. Strike C E G B on the piano in the middle register against D F# A in the second register to produce a common polychord. The B may be omitted from the first chord changing its chord quality from a major 7th to major triad. The major/minor chord contains both a major and minor triad.
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