The major third (M3) is a foundational consonant interval consisting of four semitones. It serves as the definitive marker of major tonality, providing the harmonic framework that distinguishes the major triad from the minor, diminished, and suspended chord qualities.
Construction and Diatonic Spelling
In diatonic theory, a major third must span exactly three letter names (e.g., C to E or Ab to C). While it shares a sound with the diminished fourth (C to Fb) in twelve-tone equal temperament, the spelling is dictated by its function within a scale. A major third is constructed by stacking two whole steps (M2 + M2), creating a wide, stable gap that defines the "major" character of the home key.
Enharmonic accuracy is essential for understanding voice leading. A major third implies a stable chord tone within a triad, whereas an enharmonically equivalent diminished fourth typically functions as a non-chord tone or a chromatic suspension requiring resolution. Correct naming ensures the vertical logic of the harmonic stack remains intact.
Major vs. Minor Quality
The distinction between major and minor qualities is determined by the specific placement of the third within the "frame" of the perfect fifth. In a Major Triad, the major third (4 semitones) sits at the bottom of the stack, leaving a smaller minor third (3 semitones) to reach the fifth. This creates an expansive, resonant base that the ear perceives as stable and "bright."
