Are Parallel Diminished 5ths Allowed in 4-Part Harmony?

When embarking on the fascinating journey of 4-part harmony, understanding the nuances of chord progressions and dissonance becomes crucial. Among the many tools at your disposal, the use of diminished 5ths stands out, particularly in the context of parallel diminished 5ths. Though the term 'harmony police' might evoke a sense of seriousness, no actual enforcers are chasing musicians or composers for writing certain harmonic progressions. However, mastering common practice harmony, a style rooted in the late Baroque and Classical periods, does require adherence to its established rules.

Understanding Parallel Harmony in Common Practice

Common practice harmony, with its rich history and well-documented principles, sets the foundation for much of Western music. In this context, parallel 5ths and octaves are generally avoided, as they can create an unsettling, dissonant effect when used in succession. However, one specific exception exists: parallel diminished 5ths. While they are not allowed as freely as other diminished intervals, they are permitted with proper preparation and resolution.

What Are Diminished 5ths?

A diminished 5th is an interval of three whole steps, consisting of a perfect fifth (seven semitones) diminished by one half step. In terms of enharmonic notation, it can also be represented as an augmented fourth. This interval, while dissonant, has a unique quality that can effectively add tension and interest to a piece of music. In 4-part harmony, diminished 5ths often appear as part of diminished chords or diminished seventh chords, which are rich in dissonance.

Parallel Diminished 5ths and Their Use

Parallel diminished 5ths, despite their potential for increased dissonance, can be used effectively within the framework of common practice harmony. However, their use is subject to strict rules to ensure that the dissonance is appropriately resolved. Proper preparation involves moving the voices with the diminished 5th to a consonant interval, typically a perfect fifth, fourth, or third. This process, known as 'preparation,' helps to create a sense of resolution and movement within the piece.

Resolution Techniques for Parallel Diminished 5ths

The resolution of parallel diminished 5ths is crucial for maintaining a smooth and structured harmonic flow. There are several common techniques used to resolve these intervals effectively:

Leading to a Perfect Fifth: One way to resolve a parallel diminished 5th is by moving the upper voice to the upper neighbor of a perfect fifth. This creates a sequence where the diminished 5th is followed by a perfect fifth, leading to a consonant interval. Cadential Resolution: Another method involves resolving the diminished 5th in a cadential context. In a half cadence or an authentic cadence, moving to a perfect fifth or a perfect fourth provides a satisfying resolution. Moving to a Consonant Interval: The goal is to eventually move the voices to a consonant interval, such as a major or minor third, which provides harmonic stability.

Examples and Context

To better understand the application of parallel diminished 5ths in 4-part harmony, consider the following examples. In the key of C minor, a diminished 7th chord (Cdim7) might be followed by an A major chord, creating a resolution from Cdim7 to A major via a parallel diminished 5th from the C to the A in the bass:

Cdim7 (C - Eb - Gb - Bb)    A major (A - C - E)
|: C - Eb - Gb - Bb :|     |: A - C - E :|
Bass Line: C - D - C - A

In this simple example, the bass moves from C to A, creating a parallel diminished 5th. By the time the A major chord is reached, the dissonance created by the diminished 5th is resolved, ensuring a smooth transition and maintaining harmonic stability.

Conclusion

While there are no 'harmony police' to enforce strict rules, understanding and adhering to the conventions of common practice harmony is crucial for a composer or arranger. Parallel diminished 5ths, though technically allowed, require careful preparation and resolution to be effectively integrated into a piece. By following the guidelines and techniques described above, you can master the use of parallel diminished 5ths to enhance the dissonance and complexity of your 4-part harmony.