Third

What it means

The third is the interval of three scale degrees. Common are the minor third with three semitones and the major third with four; alongside these the diminished and augmented third occur in altered contexts. Minor and major thirds count as imperfect consonances.

Musical use

The third is the building block on which the whole of Western harmony rests. Triads and seventh chords are formed from stacked thirds, and which form the third above the root takes decides the mode: a major third yields major, a minor third yields minor. A single semitone separates the two — and changes the character completely. This difference is also the basis of the Picardy third, in which a minor piece closes on a major chord.

As an imperfect consonance, the third may be led in parallel motion; the two-voice texture of melody and lower third is the most widespread form of simple polyphony. Here, major and minor thirds alternate within the diatonic scale, so that the parallels stay within the key.

Historically the third became established as a fully valid sonority only in the fourteenth and fifteenth centuries; before that, only the octave, fifth, and fourth counted as perfect consonances. With it the sound of European music changed fundamentally, for only the sound of the third makes the triad a load-bearing element.

In inversion, the third becomes the sixth.

A distinction is made between major and minor thirds; they determine the mode of triads. Thirds are imperfect consonances and are gladly led in parallel in composition, especially in folk music and duets. Chords of major-minor tonality are stacked in thirds: triads from two, seventh chords from three, ninth chords from four thirds.

Performance practice

Intonate major thirds lower and minor thirds higher than the piano plays them, and listen for the combined sound becoming calm — that is the single most effective intervention in an ensemble's intonation. And practise distinguishing the two forms of the third by ear; recognizing major and minor rests on this.