Sixth (interval)
The interval of a sixth.
What it means
The sixth is the interval spanning six scale degrees. Common are the minor with eight and the major with nine semitones; alongside these the augmented with ten occurs in the augmented sixth chord. The minor and major sixth count as imperfect consonances and may therefore be led in parallel.
Musical use
Through inversion the sixth arises from the third: from the major third the minor sixth, from the minor third the major. Its treatment in part-writing is correspondingly related — what holds for parallel thirds holds equally for parallel sixths. Tonally the sixth seems more open than the third, because the notes lie farther apart.
In the chord it appears at several places. The six-three chord (first inversion), the first inversion of the triad, carries the third and the sixth above the bass and sounds lighter than the root position. The sixte ajoutée adds a sixth instead of a seventh to a triad, which sounds soft and often appears at a close. The augmented sixth chord, with its expanded sixth, forms an especially tense turn before the dominant; depending on construction it carries the names Italian, French, and German.
Historically the fifteenth-century faux-bourdon is to be mentioned, which rests on parallel sixths and contributed to establishing this sonority.
A distinction is made between major and minor sixths; they belong among the imperfect consonances. Parallel sixths are allowed in part-writing and are gladly used in duets and wind writing. The augmented sixth forms a chord type of its own, the augmented sixth chord, which strives toward the dominant. The sixth is the inversion of the third, which is why it is closely connected with major and minor: the minor sixth inverts the major third, the major sixth the minor third.
Performance practice
Sing sixths by way of their inversion when they are difficult — a major sixth upward corresponds to a minor third downward plus an octave. With parallel sixths, match attack and length, so that they work as one sound. And lead the line back after a sixth leap, so that the phrase stays in balance.