Tritone substitution
Though examples of the tritone substitution, known in the classical world as an augmented sixth chord, can be found extensively in classical music since the Renaissance period,[1] they were not heard outside of classical music until they were brought into jazz by musicians such as Dizzy Gillespie and Charlie Parker in the 1940s,[2] as well as Duke Ellington, Art Tatum, Coleman Hawkins, Roy Eldridge and Benny Goodman.
Instead of repeating this pattern to conclude the movement, the bars that follow replace the E7 chord with a B♭7.There are similarities here with the ambivalent ending of Richard Strauss's tone poem Also sprach Zarathustra .
[5] Here, according to Richard Taruskin, "Strauss contrived an ending that seemed to die away on an oscillation between tonics on B and C, with C … getting the last word.
It would have been obvious that the C (though placed many octaves lower than its rival, in a register the ear is used to associating with the fundamental bass) was, in functional terms, making a descent to the tonic B as part of a "French sixth" chord... Rather than an ending in two keys, we are dealing with a registrally distorted, interrupted, yet functionally viable cadence on B.
Edward Sarath calls tritone substitutions a "non-diatonic practice that is indirectly related to applied chord functions... yield[ing] an alternative melodic pathway in the bass to the tonic triad.
Tritone substitutions are also closely related to the altered chord used commonly in jazz.
Jerry Coker explains: Tritone substitutions and altered dominants are nearly identical... Good improvisers will liberally sprinkle their solos with both devices.
A simple comparison of the notes generally used with the given chord [notation] and the notes used in tri-tone substitution or altered dominants will reveal a rather stunning contrast, and could cause the unknowledgeable analyzer to suspect errors.
For example, in the progression Dm7–G7–CM7, substituting D♭7 for G7 produces the downward movement of D–D♭–C in the roots of the chords, typically played by the bass.
This also reinforces the downward movement of the thirds and sevenths of the chords in the progression (in this case, F/C to F/C♭ to E/B).
The fact that a chord and its tritone substitution have the third and seventh in common is related to the fact that in 12 equal temperament, the 7:5 and 10:7 ratios are represented by the same interval, which is exactly half of an octave (600 cents) and is its own inversion.
