The minor third (m3) is a foundational consonant interval consisting of three semitones. It serves as the definitive marker of minor tonality, providing the introspective and complex harmonic framework that distinguishes the minor triad from major, augmented, and suspended chord qualities.
Construction and Diatonic Spelling
In diatonic theory, a minor third must span exactly three letter names (e.g., C to Eb or A to C). While it shares a sound with the augmented second (C to D#) in twelve-tone equal temperament, the spelling is strictly dictated by its function within a scale. A minor third is constructed by stacking a whole step and a half step (M2 + m2), creating a concentrated, stable interval that defines the "minor" character of the home key.
Enharmonic accuracy is vital for correct harmonic analysis. A minor third implies a stable chord tone within a triad, whereas an enharmonically equivalent augmented second typically functions as a melodic "stretch" leading to a resolution. Maintaining the correct spelling preserves the vertical logic of the triadic stack and ensures the scale’s structural integrity is clear to the performer.
Minor vs. Major Quality
The distinction between minor and major qualities is determined by the specific placement of the third within the "frame" of the perfect fifth. In a Minor Triad, the smaller minor third (3 semitones) sits at the bottom of the stack. This "compression" of the lower interval creates a denser, more complex harmonic profile that the ear perceives as dark, melancholic, or serious.
In a Major Triad, the intervals are inverted: the expansive major third sits at the bottom. The minor third's role in a major triad is to sit on top of the major third to "close" the fifth. However, when the minor third is placed against the root, it shifts the harmonic center of gravity. This narrowing of the interval provides the "DNA" of the minor scale; its three-semitone width creates the acoustic tension and characteristic "pathos" that defines the minor tonality compared to the "open" resonance of the major third.
