Advanced Harmonic Progressions and Chord Functions

The Descending Thirds Chord Progression

The descending thirds chord progression is a harmonic sequence where the roots of the chords move downward by the interval of a third. This movement is fundamentally defined by the root motion rather than the visual movement of the bass line. Consequently, a progression can still be classified as descending thirds even if inversions are used and the bass does not literally drop by a third. For instance, using a $I - VI$ inversion still qualifies as a descending thirds progression because the core root motion remains consistent. It is essential to remember that this progression must move downwards; moving upwards (e.g., $I - III - V$) is distinct and is not categorized as a descending thirds progression.

In this context, chord $VI$ plays a significant role. It is frequently employed because it prepares chord $IV$ with great success. When observing voice-leading theory, the transition from $VI$ to $IV$ is remarkably smooth. Traditionally, primary chords are used before secondary chords. This practice is largely a matter of convention rooted in historical applications. Through centuries of exposure to Western traditional music, this specific ordering has become naturalized to listeners—a process referred to as inculturation. While there is no immutable logical reason for this specific sequence, it remains the standard because it sounds "natural" within the established musical culture.

Historical Context and Musical Stock Phrases

During the 1700s, music composition often relied on what could be described as "stock phrases." Much like contemporary stock images or audio clips available on the internet today, musicians of the 18th century utilized standardized melodic and harmonic formulas. Composition and improvisation during this era often involved taking these established progressions and adding diminutions—melodic decorations or subdivisions of longer notes. One of the most famous historical examples of this type of stock phrase is Pachelbel's Canon. These progressions, including the descending thirds and the circle of fifths, formed the backbone of musical structure through the Classical and Romantic eras.

The Circle of Fifths and Chord Functions

The circle of fifths is a progression where the roots of the chords drop by a fifth or ascend by a fourth. A descending fifth and an ascending fourth are essentially the same note in terms of pitch class. While many progressions in the circle of fifths begin on chord $I$, it is actually more common in classical literature to begin the sequence on chord $VI$. A very well-known specific application of the circle of fifths is the $II - V - I$ progression.

To understand these progressions, it is helpful to categorize chords by their harmonic functions. Chord $I$ and chord $VI$ serve a tonic function, providing a sense of rest or home base. Chord $IV$ and chord $II$ serve a predominant function, meaning they are used to lead toward the dominant. Chord $V$ and chord $VII$ serve a dominant function, creating the tension that requires resolution back to the tonic. Chord $II$ often acts as a substitute for chord $IV$ or is used to prolong the predominant function. An example of this is found in Beethoven's Violin Sonata in F Major, where chord $II$ is utilized effectively within the harmonic structure.

Voice Leading Challenges in Minor Keys

When writing in a minor key, specific care must be taken regarding scale degrees 6 and 7. If these degrees are situated adjacently as the highest members of the chords, they can create an interval known as an oblique second (referring to the melodic tension of the augmented second in the harmonic minor scale). To resolve this issue, composers typically have two options. The first is to alter the chord type, such as altering chord $IV$ or $II$ by raising scale degree 6. The second option is to revoice the chords so that scale degrees 6 and 7 are not adjacent in the upper voice, thereby avoiding the problematic interval.

Chord III: Tonic and Dominant Substitutions

In a major key, chord $III$ has a minor sonority. It can function as a substitute for chord $I$, carrying a tonic function, particularly when used in the first inversion ($III^6$). However, chord $III$ can also function as a dominant substitute. In a sequence such as $V - III^6 - I$, the chord $III^6$ maintains a strong dominant sonority and functions accordingly within the context. This chord is often used to substitute for chord $I^6$ in progressions like $I - III - IV$ or $VI - III - IV$.

This specific progression—$I - III - IV$—is often called the descending leading tone progression or the descending Riemannian progression. It is characterized by the roots moving in steps and facilitates a leap of a third in the voice leading. In minor keys, the use of chord $III$ varies based on its intended function. If it is substituting for chord $I$, the leading tone remains unraised, resulting in a major chord ($III$). If it is intended to carry a dominant function, it should be used in its augmented form by raising the leading tone. In this augmented state, it is standard practice to use the chord in its first inversion ($III^+{^6}$) to maintain proper sonority (for example, using pitches like $E\flat$, $G$, and $B\natural$).

Chord VII and Tritone Resolution

Chord $VII$ exclusively serves a dominant function and is used either after chord $V$ or as a total replacement for it. It can be used to create a perfect cadence ($VII - I$). In both major and minor keys, chord $VII$ contains a tritone (an augmented fourth or diminished fifth). Specifically, when used in first inversion, the two members in the right hand will form a tritone which must resolve according to strict rules. The leading tone within chord $VII$ must resolve upward to the tonic ($I$). Additionally, the fifth of the chord must always resolve downward by one scale step. This resolution is mandatory regardless of whether the piece is in a major or minor key to ensure proper harmonic motion.

Questions & Discussion

Student: So you're saying it's more common for when using the circle of fifths to start from the sixth? Instructor: Yes. That's not wrong. You can start on four or one or two, but starting on six was more frequently used in the literature than starting on one.

Student: I missed the last class. [Name] told me to send a voice note summarizing the work for five minutes. Instructor: That sounds like a waste of time. We didn't do much last week; it was just basic work.

Student Discussion: The students and instructor briefly discussed attendance requirements for the semester and the year, specifically mentioning the "MGS" and "MOM" modules. A student expressed frustration with rhythmic and melodic dictation/exercises, noting they would likely need to attend tutorials in the next semester due to the difficulty of the material. There was also a brief mention of a grade or mark that might be high enough to avoid extra tutorials.