Augmented-Sixth Chords vs. Tritone Substitutes

Abstract

Augmented-sixth chords and tritone substitutes have long been recognized as enharmonically equivalent, but to date there has been no detailed and systematic examination of their relationships from the perspectives of both classical and jazz theories. Augmented-sixth chords and tritone substitutes share several structural features, including pitch-class content, nonessential fifths, underdetermined roots, structural positioning, and two possible harmonic functions: pre-dominant or dominant. A significant distinction can be made between these two chord classes on the basis of their behaviors: they differ in their voice-leading conventions of contrary vs. parallel, normative harmonic function as dominant preparation vs. dominant substitute, and enharmonic reinterpretation as modulatory pivot vs. dual-root dominant approaching a single resolution. In light of these differences, examples of both augmented-sixth chords and tritone substitutes can be identified in both the jazz and art-music repertoires.