Definition
In Western tonal music theory, an augmented seventh refers to an interval that spans seven letter names and is one semitone larger than a major seventh. It comprises twelve semitones, making it enharmonically equivalent to a perfect octave. The interval is notated as an augmented seventh (A7) and is typically represented in music notation by raising the upper note of a major seventh by a chromatic semitone (e.g., C to B♯).
Construction
- Major seventh: 11 semitones (e.g., C to B)
- Augmented seventh: 12 semitones (e.g., C to B♯)
Because the octave also contains twelve semitones, the augmented seventh is enharmonically identical to a perfect octave, though its spelling and theoretical function differ.
Notation and Symbolism
- Interval symbol: A7 (or “aug 7”)
- In chord symbols, an augmented seventh may be indicated in contexts such as “C7♯5” where the fifth is raised, but the term augmented seventh specifically describes the interval rather than chord quality.
Theoretical Context
- Functional Role: The augmented seventh is rarely used as a standalone interval in melodic writing because its pitch class coincides with the octave, creating practical redundancy. However, its theoretical distinction is important in harmonic analysis, particularly when differentiating between voice-leading options that involve chromatic alteration of the seventh scale degree.
- Enharmonic Equivalence: Because it is enharmonically equivalent to the perfect octave, the augmented seventh often appears in theoretical discussions of enharmonic modulation and reinterpretation of pitch relationships.
Examples
- From C to B♯ (C–B♯) – an augmented seventh spanning twelve semitones.
- From F♯ to E♯ (F♯–E♯) – another instance, where the upper note is raised a chromatic semitone above the major seventh interval of F♯ to E.
Historical Usage
The concept of augmented intervals, including the augmented seventh, emerged in the common practice period as composers explored chromatic alteration of diatonic intervals. The term appears in 19th‑century music theory treatises (e.g., those by Heinrich Schenker and Hugo Riemann) to describe chromatically altered sevenths within complex harmonic progressions.
Practical Considerations
- Voice Leading: When a voice moves from the leading tone to the tonic in a minor key, composers may spell the leading tone a semitone higher (e.g., B♮–C) to create a diminished seventh. Raising this to an augmented seventh (B♯–C) serves specific chromatic functions, such as facilitating enharmonic modulation.
- Performance: Since the pitch produced by an augmented seventh is identical to an octave, performers treat it as an octave in terms of intonation, while acknowledging the distinct notational spelling for analytical purposes.
References in Standard Works
- Aldwell, Edward, and Carl Schachter. Harmony and Voice Leading. 3rd ed.
- Brainerd, Richard S. An Introduction to Music Theory.
- Piston, Walter. Harmony.
These texts discuss augmented intervals, including the augmented seventh, within the broader framework of chromatic harmony and intervallic analysis.