Hypomixolydian
Plagal variant of the Mixolydian mode.
What it means
Hypomixolydian is the plagal form of the Mixolydian mode and the eighth of the church tones. Finalis and pool of notes agree with the Mixolydian mode; the difference lies in the ambitus, which extends substantially below the finalis, and in the reciting tone.
Musical use
In the order of the eight church tones, each authentic mode forms with its plagal counterpart a pair that shares the same finalis. For liturgical practice the distinction matters, because each of the eight tones has a psalm formula of its own: the reciting tone and the cadential turns follow from it, and an antiphon in the eighth tone demands a different formula from one in the seventh, although both have the same finalis.
Characteristic of the Mixolydian stock is the low seventh degree, which lies a whole tone below the finalis. As a result the leading tone is missing, and the melody does not aim with the inevitability that is familiar from later major–minor tonality. In the plagal form this trait recedes somewhat further, because the centre of gravity of the motion lies lower.
For present-day editions the question of musica ficta arises here as with all modes: whether and where the seventh degree was raised in cadences is to be decided in the individual case and can be recognized in scholarly editions by small accidentals above the staff.
Hypomixolydian is the eighth and last of the medieval church tones. The finalis is G as with authentic Mixolydian, the tonal space runs from D to D, and the reciting tone lies on C. Many well-known chants of the Gregorian repertoire, including hymns and antiphons, stand in this mode. In psalmody the eighth tone has a series of typical closing formulas.
Performance practice
Determine finalis and ambitus before you decide on the performance — the register tells you where the melody breathes. Avoid hearing the music into a major or minor melody; the missing leading-tone tension is no deficiency but the character. And settle in the ensemble beforehand which cadential notes are raised.