Card 67 / 118
V. Chromaticism
Common-Tone Chords (CTº7 &amp; CT+6)

### Core Idea

The common-tone diminished seventh chord (CTo7) and common-tone augmented sixth chord (CT+6) have the same pitches as [viio7](https://viva.pressbooks.pub/openmusictheory/chapter/leading-tone-chord/) and [Ger+6](https://viva.pressbooks.pub/openmusictheory/chapter/augmented-sixth-chords/) 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.

### Explanation

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.

Chapter Playlist

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.Ger+6 does have a common tone with \mathrm{cad.^6_4}, but not with the dominant chord that follows it, which is the actual destination chord. 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.The neighbor effect is somewhat less striking when embellishing a minor triad, since the third of the minor triad creates a second common tone with the embellishing chord.

Deriving a CTo7 chord from multiple neighbor tones

Example 1 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 \mathrm{vii}^{\circ}\begin{smallmatrix}4\\2\end{smallmatrix}/\mathrm{iii}. The problem is, there is no iii chord to be found, so that analysis wouldn't represent this music accurately.

Example 1. Multiple neighbor tones can come together to form a CTo7 chord.

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 Example 1 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).

You can create a CTo7 chord by going through the following procedure:

Find the root of the chord you want to embellish (this will be the common tone).

Find the upper and lower neighbors to the fifth of the embellished chord.

The upper neighbor will involve a whole step (major second).

The lower neighbor will involve a half step (minor second).

Find the lower neighbor (minor second in particular) of the third of the embellished chord.

Recognizing CTo7 when analyzing

Finding this chord in context involves being aware of all fully diminished seventh chords. For each fully diminished seventh chord you find, determine if it has a common tone with the chord it resolves to. If it doesn't, then it should be some form of viio7 or an applied viio7 chord. If it does have a common tone, then it is a CTo7 chord. Remember, though, that if the fully diminished seventh chord is followed by a \mathrm{cad.^6_4}, you need to look past the \mathrm{cad.^6_4} embellishment to its resolution to determine if there is a common tone (see Example 2).

Example 2. The \mathit{cad.^6_4} can produce a common-tone that does not relate to CTo7 chords.

Resolving CTo7 to V7

When a CTo7 chord resolves to V7 instead of a V triad, the fifth of the V7 chord (re, \hat2) is not doubled. Instead, the voice that moves from re to mi (\hat2–\hat3) in the CTo7 will continue upward to fa (\hat4) in the V7. Example 3 compares the re–mi–re (\hat2-3-2) line of the resolution to V with the re–mi–fa (\hat2-\hat3-\hat4) line of the resolution to V7.

Example 3. Comparison of a complete neighbor CTo7 that resolves to V and to V7.

CTo7 with incomplete neighbors

The CTo7 is often preceded and followed by the same chord, producing complete neighbor tones in the pattern x−CTo7−x. However, the CTo7 chord may be preceded by a different chord instead (y−CTo7−x), and in such cases, one or more of the neighbor tones will be incomplete.

Example 4 demonstrates this situation. Notice that the first CTo7 is surrounded by different chords: first I, then \mathrm{V^4_3}. The arrow shows which chord the CTo7 is embellishing. Just like in complete-neighbor CTo7 contexts, the embellished chord is the one to focus on, not the preceding chord. This progression is an elaboration of the \m

### Key Quote

Common-tone chords are so named because they contain the root of (i.e., have a common tone with) the chord being embellished.

### Application Scenarios

Example 1 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 \mathrm{vii}^{\circ}\begin{smallmatrix}4\\2\end{smallmatrix}/\mathrm{iii}. The problem is, there is no iii chord to be found, so that analysis wouldn't represent this music accurately.

Example 1. Multiple neighbor tones can come together to form 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 Example 1 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).

Finding this chord in context involves being aware of all fully diminished seventh chords. For each fully diminished seventh chord you find, determine if it has a common tone with the chord it resolves to. If it doesn't, then it should be some form of viio7 or an [applied viio7](https://viva.pressbooks.pub/openmusictheory/chapter/tonicization/) chord. If it does have a common tone, then it is a CTo7 chord. Remember, though, that if the fully diminished seventh chord is followed by a \mathrm{cad.^6_4}, you need to look past the \mathrm{cad.^6_4} embellishment to its resolution to determine if there is a common tone (see Example 2).

### Key Terminology

diminished seventh chord dominant seventh chord seventh chord minor triad major triad common tone function measure texture tonic final triad

### Related Links

viio7 Ger+6 Chapter Playlist viio7 Ger+6 applied viio7 The Sycamore Etude in F minor.pdf.docx
Source: https://viva.pressbooks.pub/openmusictheory/chapter/common-tone-chords/
中文翻译将在推送时生成