Augmented-Sixth Chords vs. Tritone Substitutes

[1] Augmented-sixth chords and tritone substitutes have long been recognized as enharmonic equivalents. Although this relationship has been briefly noted in a few recent textbooks, to date no in-depth comparison from the standpoint of both classical and jazz theories exists in the scholarly literature.(1) A close consideration of commonalities and differences in structure and function can lead to a richer understanding of both chord types. Their relationship is generally described as two different perspectives on the same syntactic structure: in classical terminology, an augmented-sixth chord; in jazz, a tritone substitute. An obvious distinction in their typical contexts is that augmented sixths are a compositional characteristic of art music from the 18th and 19th centuries, while tritone substitution has been a performance practice in improvised jazz since the bebop era (mid-1940s) or slightly earlier. For purposes of comparison to augmented-sixth chords, I have used examples of tritone substitutes from published arrangements of jazz standards.[2] The potential harmonic roles of these two chord types are similar: they function either as dominant preparations or as substitute dominants. As I will demonstrate, however, the two chord classes result not simply from different stylistic perspectives on the same musical construct, but from significant differences in normative harmonic function, conventional voice-leading procedures, and possibilities for enharmonic reinterpretation, as reflected in compositional practice. Thus both chord types can be distinguished in both art-music and jazz repertoires.Augmented-Sixth Chords[3] As is well known, the fundamental principle of augmented-sixth chords is that the defining interval resolves outward in contrary motion to an octave. In the most common configuration, scale degrees and act as double leading tones facilitating a strong arrival on V, as in Example 1, from the opening of Beethoven's Symphony No. 5 (1808). In dominant-function augmented sixths, scale degrees and resolve to the tonic, as in Example 2, the ending of Schubert's C-major String Quintet (1828). Since the chord in this instance is a French augmented sixth, it can also be interpreted as V7 with a flat fifth in the bass. More rarely, the octave resolution of the augmented-sixth interval is to the third of the tonic triad, which Daniel Harrison calls the 'plagal' augmented sixth because of the 4-3 bass motion, or to the fifth of the tonic triad, known as a common-tone augmented sixth.(2)Example 1. Augmented Sixth as Pre-Dominant:Beethoven Symphony No. 5, first movement, mm. 18-21Example 2. Augmented Sixth as Dominant:Schubert, String Quintet in C, fourth movement, mm. 425-431(click to enlarge and see the rest)Tritone Substitutes[4] The basis of tritone substitution is that the active interval of the tritone in any dominant-seventh chord is shared by another dominant-seventh chord whose root is a tritone away, as shown in Example 3. These harmonies function interchangeably because the tritone is inversionally symmetrical. Thus the chordal thirds and sevenths, which form the tritones, map onto each other, producing smooth voice-leading in either case. In theory, tritone substitutes replace dominants, and this is indeed their most usual function. In practice, however, tritone substitution is also applied to chords without a tritone-typically dominant preparations such as vi and ii, either preserving the original chord qualities or recasting them as dominant sevenths. Both chords of a ii-V progression are commonly substituted, as in Example 4, the end of the A phrase from Ellington's "Satin Doll" (1953). In this instance the minor quality of the ii7 chord has been retained, and the conventional ii-V-I (Dm-G-C) has been replaced by vi-II-I (Am-D-C).Example 3. Tritone Substitution and ResolutionExample 4. …