The third is the heart of tonal music. It determines whether a piece will be “bright” (major) or “dark” (minor). All Western classical and popular music rests on this distinction.
Parallel thirds often create an “angelic duet” effect — doubled voices sound gentle and sweet. Characteristic examples include the Italian operas of Bellini and Donizetti, where prima donnas sang together in thirds. In folk music, thirds are the foundation of ensemble singing (Ukrainian songs, the polyphonic choral singing of Georgia).
A distinction is drawn between the major third (two tones) and the minor third (a tone and a semitone): the first forms a major triad, the second a minor one, so it is the third that “decides” a chord's mood. The tertian principle of construction (stacking thirds on top of one another) underlies all classical harmony — from the triad to many-note jazz chords. In ensemble singing, parallel thirds create a natural, “folk” two-part texture.
