V. Chromaticism
62
Brian Jarvis
Key Takeaways
- The common-tone diminished seventh chord (CTo7) and common-tone augmented sixth chord (CT+6) have the same pitches as viio7 and Ger+6 but a different function: to embellish the upcoming chord (a major triad or dominant seventh chord, typically I or V).
- Common-tone chords are so named because they contain the root of (i.e., have a common tone with) the chord being embellished.
- In a four-voice texture, the fifth of the embellished chord is often doubled.
The common-tone diminished seventh chord (CTo7) and common-tone augmented sixth chord (CT+6) represent a completely different usage of two chords with which you are already familiar: viio7 and Ger+6. Whereas these chords typically function in a more progressive harmonic context, when employed as common-tone chords, they serve a purely embellishing function and are the result of the culmination of multiple simultaneous neighbor tones. Common-tone chords share a common tone with the chord being embellished, whereas viio7 and Ger+6 do not.[1] You can expect that the embellished chord will either be a major triad or sometimes a dominant seventh chord, and the Roman numeral will be either I or V. Minor triads can be embellished in this way too, but this is far less common, so this chapter will focus on major triads and dominant seventh chords only.[2]
Deriving a CTo7 chord from multiple neighbor tones
shows how a CTo7 chord is produced through the layering of simultaneous neighbor tones. In the first system, each three-measure unit applies a single neighbor tone to the tonic chord. The second system uses two neighbor tones at a time—here, each three-measure unit is still a single tonic chord throughout, just with two neighbors instead of one. The final system shows all three neighbors combined into a single chord: a CTo7 chord. Notice that the chord in the middle has a fully diminished quality, but if you try to wrangle it into being some type of viio7 chord, you’ll come up with [latex]\mathrm{vii}^{\circ}\begin{smallmatrix}4\\2\end{smallmatrix}/\mathrm{iii}[/latex]. The problem is, there is no iii chord to be found, so that analysis wouldn’t represent this music accurately.
https://musescore.com/user/32728834/scores/6224659/embed
Creating a CTo7 chord
Creating a CTo7 chord is a little different from spelling a traditional chord because it doesn’t have a root (similar to augmented sixth chords). To build one, focus on the neighboring aspect of the chord. Look again at the final system of and notice how the notes of the CTo7 chord relate to the tonic chord. The root of the tonic chord is also in the CTo7 chord—that’s the common tone. The fifth of the resolving chord is doubled because it is embellished by both upper and lower neighbor tones (G–A–G and G–F♯–G). Finally, the third of the tonic chord is embellished by its lower neighbor a half step below (E–D♯–E). Finally, look at how the third of the chord is embellished by its lower neighbor (E–D♯–E).
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