The Hypolydian mode, literally meaning "below Lydian", is the common name for the sixth of the eight church modes of medieval music theory (Powers 2001b). The name is taken from Ptolemy of Alexandria's term for one of his seven tonoi, or transposition keys (Powers 2001a). This mode is the plagal counterpart of the authentic fifth mode.
In medieval theory the Hypolydian mode was described either as (1) the diatonic octave species from C to the C an octave higher, divided at the final F (C–D–E–F + F–G–A–B–C) or (2) a mode with F as final and an ambitus from the C below the final to the D above it. The third above the final, A—corresponding to the reciting tone or "tenor" of the sixth psalm tone—was regarded as having an important melodic function in this mode. The sequence of intervals was therefore divided by the final into a lower tetrachord of tone-tone-semitone, and an upper pentachord of tone-tone-tone-semitone. However, from as early as the time of Hucbald the Hypolydian mode—even more than the corresponding authentic mode, the Lydian—was characterized by the predominance of B♭ instead of B♮ as the fourth degree above the final (Powers 2001b). The melodic centering on F and A, as well as the use of B♭ instead of B♮, is illustrated in the accompanying example from the Requiem Mass introit, "Requiem aeternam".
Finer distinctions among the scale degree are sometimes made, with the D below the final called the "mediant", the lowest note, C, the "participant" (a tone functioning as an auxiliary to the mediant), the G, B, and B♭ the "conceded modulations" (subsidiary degrees), and the lowest C, the final, F, and (rarely) the D the "absolute initials" (Rockstro 1880, 342).
- Powers, Harold S. 2001a. "Dorian". The New Grove Dictionary of Music and Musicians, second edition, 29 vols., edited by Stanley Sadie and John Tyrrell, 12:38. London: Macmillan Publishers. ISBN 978-0-333-60800-5.
- Powers, Harold S. 2001b. "Hypolydian". The New Grove Dictionary of Music and Musicians, second edition, 29 vols., edited by Stanley Sadie and John Tyrrell, 12:38. London: Macmillan Publishers. ISBN 978-0-333-60800-5.
- Rockstro, William Smyth. 1880. "Modes, the Ecclesiastical". A Dictionary of Music and Musicians (A.D. 1450–1880), by Eminent Writers, English and Foreign, vol. 2, edited by George Grove, D. C. L., 340–43. London: Macmillan and Co.
- Gissel, Siegfried. 1997. "Glareans Tonarten Lydius und Hypolydius und ihre Berücksichtigung durch die Theoretiker/Komponisten bis etwa 1650". Musica Disciplina 51:73–102.
- Marzi, Giovanni. 1973. "Il papiro musicale dell' Oreste di Euripide (Pap. Vindob. G 2315)". In Scritti in onore di Luigi Ronga, 315–29. Milan and Naples: Ricciardi.
- Ruhnke, Martin. 1992a. "Glareans lydischer und hypolydischer Modus bei Dulichius". In Festschrift Hubert Unverricht zum 65. Geburtstag, edited by Karlheinz Schlager, 221–29. Eichstätter Abhandlungen zur Musikwissenschaft 9. Tutzing: Hans Schneider. ISBN 3-7952-0719-3.
- Ruhnke, Martin. 1992b. "Vorbereitung der Edition eines Jahrgangs Evangelien-Motetten von Philipp Dulichius". In Musik des Ostens: Ostmittel-, Ost- und Südosteuropa. XII, edited by Hubert Unverricht, 95–113. Musik des Ostens: Ostmittel-, Ost- und Südosteuropa 12. Kassel: Bärenreiter. ISBN 3-7618-1066-0.