Tritone substitution
The tritone substitution is one of the most common chord substitutions found in jazz and was the precursor to more complex substitution patterns like Coltrane changes. Tritone substitutions are sometimes used in improvisation—often to create tension during a solo. 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, they were not heard until much later in jazz by musicians such as Dizzy Gillespie and Charlie Parker in the 1940s, as well as Duke Ellington, Art Tatum, Coleman Hawkins, Roy Eldridge and Benny Goodman. For example, using D♭ major instead of G major in the key of C major is a tritone substitution (D♭ is a tritone away from G).
In tonal music, a conventional perfect cadence consists of a dominant seventh chord followed by a tonic chord. For example, in the key of C major, the chord of G7 is followed by a chord of C. A common variant of this progression would be to replace the dominant seventh chord with a dominant chord that has its root a tritone away from the original:
Franz Schubert’s String Quintet in C major concludes with a dramatic final cadence that uses the third of the above progressions. The conventional G7 chord is replaced in bars 3 and 4 of the following example with a D♭7 chord, with a diminished fifth (G♮ as the enharmonic equivalent of A
); a chord otherwise known as a ‘French sixth’:
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