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The common-tone diminished seventh chord (CT^o7) and common-tone augmented sixth chord (CT⁺⁶) represent a completely different usage of two chords with which you are already familiar: vii^o7 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 vii^o7 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 CT^o7 chord from multiple neighbor tones

Example 1 shows how a CT^o7 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 CT^o7 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 vii^o7 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.

Example 1. Multiple neighbor tones can come together to form a CT^o7 chord.

Creating a CT^o7 chord

Creating a CT^o7 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 CT^o7 chord relate to the tonic chord. The root of the tonic chord is also in the CT^o7 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 CT^o7 chord by going through the following procedure:

1. Find the root of the chord you want to embellish (this will be the common tone).
2. 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).
3. Find the lower neighbor (minor second in particular) of the third of the embellished chord.

Recognizing CT^o7 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 vii^o7 or an applied vii^o7 chord. If it does have a common tone, then it is a CT^o7 chord. Remember, though, that if the fully diminished seventh chord is followed by a [latex]\mathrm{cad.^6_4}[/latex], you need to look past the [latex]\mathrm{cad.^6_4}[/latex] embellishment to its resolution to determine if there is a common tone (see Example 2).

Example 2. The [latex]\mathit{cad.^6_4}[/latex] can produce a common-tone that does not relate to CT^o7 chords.

Resolving CT^o7 to V⁷

When a CT^o7 chord resolves to V⁷ instead of a V triad, the fifth of the V⁷ chord (re, [latex]\hat2)[/latex] is not doubled. Instead, the voice that moves from re to mi [latex](\hat2–\hat3)[/latex] in the CT^o7 will continue upward to fa [latex](\hat4)[/latex] in the V⁷. Example 3 compares the re–mi–re [latex](\hat2-3-2)[/latex] line of the resolution to V with the re–mi–fa [latex](\hat2-\hat3-\hat4)[/latex] line of the resolution to V⁷.

Example 3. Comparison of a complete neighbor CT^o7 that resolves to V and to V⁷.

CT^o7 with incomplete neighbors

The CT^o7 is often preceded and followed by the same chord, producing complete neighbor tones in the pattern x−CT^o7−x. However, the CT^o7 chord may be preceded by a different chord instead (y−CT^o7−x), and in such cases, one or more of the neighbor tones will be incomplete.

Example 4 demonstrates this situation. Notice that the first CT^o7 is surrounded by different chords: first I, then [latex]\mathrm{V^4_3}[/latex]. The arrow shows which chord the CT^o7 is embellishing. Just like in complete-neighbor CT^o7 contexts, the embellished chord is the one to focus on, not the preceding chord. This progression is an elaboration of the [latex]\mathrm{I-V^4_3-I^6}[/latex] progression that you’ve encountered with tonic prolongation but with CT^o7 chords filling in the space between each chord providing a much more colorful version of what was a rather simple progression. When listening to Example 4, try to hear the underlying [latex]\mathrm{I-V^4_3-I^6}[/latex] progression as its underlying model.

Example 4. A tonic expansion pattern elaborated by two different CT^o7 chords comprising incomplete neighbors.

Creating a CT⁺⁶ chord

The other category of common-tone chords you’ll encounter (especially in music of the later 19^th century) is a German augmented sixth chord (Ger⁺⁶) that functions, like the CT^o7, as an embellishing chord. The effect is similar to the CT^o7, but a little darker, because all three neighbor tones are chromatic in this version instead of just two: a CT⁺⁶ chord uses a minor second neighbor above the fifth of the chord instead of a major second.

1. Find the root of the chord you want to embellish (this will be the common tone). Tonic is the most common harmony to embellish with a CT⁺⁶ chord.
2. Find the upper and lower neighbors to the fifth of the embellished chord. Both will be a half step.
3. Find the lower neighbor (minor second in particular) of the third of the embellished chord.

When a CT⁺⁶ chord is embellishing a I chord (which is usually the case), you can think in terms of solfège to find the notes. The solfège is the same as for a Ger⁺⁶ chord: le–do–me/ri–fi [latex](\downarrow\hat6-\hat1-\downarrow\hat3/\uparrow\hat2-\uparrow\hat4)[/latex]. Because this chord is most often found in major keys, ri [latex](\uparrow\hat2)[/latex] would be a better spelling than me [latex](\downarrow\hat3)[/latex].

Example 5 below shows both chords (CT^o7 and CT⁺⁶) for comparison. Note that the common tone is not always the bass note and that the augmented sixth interval may be inverted to become a diminished third instead, but the chord is typically labeled CT⁺⁶ in both contexts.

Example 5. Comparison between a CT^o7 and CT⁺⁶ chord, both as complete neighbors.

Musical Examples

The introduction to Scott Joplin’s “The Sycamore” shows how a CT^o7 can be sandwiched between a [latex]\mathrm{cad.^6_4}[/latex] and its resolution to V⁷ (Example 6). It appears as though the bass is moving from D to F♯ and back to D, but the F♯ is not related to the functional bass of the passage.

Example 6. Scott Joplin, “The Sycamore,” mm. 1–4. A CT^o7 is introduced between the start of the [latex]\mathit{cad.^6_4}[/latex] and its resolution to V⁷

In Example 7, Frederic Chopin uses a CT⁺⁶ in m. 8 at the conclusion of a parallel period that is occuring over a tonic pedal. This instance is a common-tone chord that is an incomplete neighbor, because the chord before it in measure 7 is V7 above the tonic pedal. Notice also that Chopin spelled the chord with a C♭ instead of a B♮. Spelling variants do happen with common-tone chords, but the B♮ spelling in this case would have clarified the neighboring function of that note with surrounding Cs in that voice.

Example 7. Frederic Chopin, Etude in F minor, Op. 10, no. 9, mm. 1–9. A CT⁺⁶ occurs in the context of a tonic pedal in m. 8.

1. Common-Tone Chords (.pdf, .docx.) Asks students to spell common tone chords, realize figured bass, complete 4-part voice leading with Roman numerals, and analyze a musical excerpt. Access audio (excerpt begins at 0:25).