Definition
In music theory, a third inversion refers to the arrangement of a seventh chord in which the seventh degree of the chord is placed in the lowest pitch (the bass). The term is used to describe the specific voicing of chords that contain four distinct pitch classes (root, third, fifth, and seventh) when the seventh is the bass note.
Context in Music Theory
- Triads (three‑note chords) have only two inversions: the first inversion (third in the bass) and the second inversion (fifth in the bass).
- Seventh chords (four‑note chords) add a third inversion, because the seventh can be placed in the bass. This provides four distinct chord positions: root position, first inversion, second inversion, and third inversion.
Notation and Voice Leading
- In Roman‑numeral analysis, a third‑inverted seventh chord is often marked with a “⁷⁶⁵” superscript (e.g., V⁷⁶⁵) to indicate the specific intervallic structure of the chord above the bass.
- In figured bass, the figure “6‑5” (or “6/5”) is used to denote a third‑inverted seventh chord.
- In lead‑sheet or chord‑symbol notation, the inversion may be indicated by a slash and the bass note (e.g., C⁷⁄B♭) or by an explicit layout of the chord tones.
Examples
- C⁷ (C–E–G–B♭) in third inversion: B♭ is the bass, with the remaining chord tones (C, E, G) above it.
- Dominant seventh chord in a cadential progression: In the key of F major, the V⁷ chord (C⁷) often appears in third inversion (B♭ in the bass) to facilitate a smooth stepwise motion to the tonic chord (F major).
Functional Significance
- The third inversion weakens the sense of tonal stability because the seventh, which creates a dissonant interval with the bass, occupies the lowest position. This instability is frequently employed to generate forward motion toward a resolution.
- In harmonic progressions, the third‑inverted seventh chord commonly resolves to a chord whose root is a half step above the bass of the inverted seventh chord, creating a descending bass line (e.g., B♭ → C in the example above).
Historical and Pedagogical Usage
- The concept dates back to Baroque theorists such as Johann Joseph Fux, who described chord inversions as part of counterpoint practice.
- Modern theory textbooks (e.g., Tonal Harmony by Kostka & Payne) treat the third inversion as a standard element of functional harmony curricula.
Related Concepts
- First inversion: Third of the chord in the bass.
- Second inversion: Fifth of the chord in the bass.
- Root position: Root of the chord in the bass.
- Inversions of extended chords: Chords beyond seventh chords (e.g., ninth, eleventh) have additional inversions, but the term “third inversion” specifically applies to seventh chords.
References
- Kostka, S., & Payne, D. Tonal Harmony, 8th ed., McGraw‑Hill Education.
- Rameau, J.-P. Traité de l'harmonie, 1722.
- Wikipedia contributors. “Inversion (music).” Wikipedia, The Free Encyclopedia.
This entry provides an objective overview of the term “third inversion” as it is recognized in established music theory literature.
