Unlocking the Augmented Third: A Rare and Expressive Interval

Summary:

Dive into the augmented third, a rare but powerful interval in music theory. This article demystifies its structure, its enharmonic relationship with the perfect fourth, and its crucial role in guiding voice leading and creating chromatic tension in sophisticated harmonies.

Keywords:

augmented third, music theory, intervals, harmony, dissonance, enharmonic equivalent, chromaticism, voice leading, chord function, music notation

Introduction:

Have you ever encountered a musical interval that sounds familiar but looks strange on the page? Welcome to the intriguing world of the augmented third. While far less common than its major or minor cousins, the augmented third is a potent tool used by composers to inject intense expression and create sophisticated forward motion. Understanding this interval unlocks a deeper appreciation for the logic of chromatic music and the subtle power of musical notation.

Definition: What Is an Augmented Third?

An augmented third is an interval created by widening a major third by one chromatic semitone. Since a major third spans four semitones (like C to E), an augmented third spans five semitones (like C to E-sharp).

Sonically, it is identical to a perfect fourth, which also spans five semitones (like C to F). This relationship is called "enharmonic equivalence"—they sound the same but are spelled differently. So why use a different spelling? Because musical spelling signals musical function. An augmented third implies a very specific and powerful tension that resolves by expanding outward to a perfect fifth. The sharped note acts as a temporary leading tone, pulling irresistibly upward.

Examples of the Interval

Example 1: Isolated Augmented Thirds

The following examples show several augmented thirds. Notice that while each one spans five semitones (like a perfect fourth), their spelling as a third (e.g., from a 'C' to some kind of 'E') is what defines them.

Practical Application: Voice Leading in Action

The augmented third is a creature of counterpoint and voice leading. You won't find it in simple diatonic chords, but it emerges when composers use chromaticism to guide melodic lines smoothly and logically. Its primary function is to create and resolve tension. By spelling an interval as C to E-sharp instead of C to F, a composer gives the performer a crucial instruction: the E-sharp is not a stable note, but an active one that must resolve upwards to F-sharp.

This creates a powerful sense of expansion. The example below shows this classic resolution in a two-voice context. The augmented third (C4 and E#4) on beat three resolves outward to a perfect fifth (B3 and F#4).

Historical Context

This type of deliberate voice leading has been a cornerstone of Western music for centuries. You can find the logic of the augmented third at play in the complex counterpoint of J.S. Bach, where independent melodic lines momentarily clash to create expressive dissonances that resolve perfectly. As music moved into the Classical and Romantic eras, composers like Mozart and Wagner used such chromatic devices with increasing frequency to heighten emotional drama. While the logic is similar to that found in the famous "augmented sixth" chords (which also feature chromatic tones resolving outward) , the augmented third is a more specific and linear event, born from the collision of two melodic lines.

Fun Facts

The augmented third holds a few interesting properties. Its inversion is a **diminished sixth**. For example, if you invert C-E# (an augmented third), you get E#-C (a diminished sixth). This relationship highlights its dissonant and unstable nature. It's also so context-dependent that many beginner theory textbooks omit it entirely, making its discovery a rite of passage for students delving into advanced harmony and counterpoint.

Conclusions

The augmented third is far more than a theoretical curiosity; it's a testament to the power of musical notation to convey intent. While it sounds identical to a perfect fourth, its spelling reveals its true purpose: to create intense, localized chromatic tension that demands an outward resolution. By learning to recognize the augmented third, you learn to read between the notes, appreciating the subtle logic of voice leading and the profound expressive potential of dissonance. The next time you see an unusual sharp or flat, ask yourself: is this creating a rare interval, and what direction is it telling the music to go?

References:

1. Kostka, S., & Payne, D. (2017). Tonal Harmony: With an Introduction to Twentieth-Century Music. McGraw-Hill Education.

2. Aldwell, E., & Schachter, C. (2011). Harmony and Voice Leading. Schirmer.

3. Laitz, S. G. (2015). The Complete Musician: An Integrated Approach to Theory, Analysis, and Listening. Oxford University Press.