(Gk.: ‘eightfold sound’)
The system of the eight ‘church modes’ (the ‘musical’ oktōēchos) in the medieval Latin, Byzantine, Slavonic, Syrian, Armenian and Georgian repertories of Christian liturgical chant. Also, by association, the practice of grouping chants by mode (the ‘calendric’ oktōēchos) so that they can be sung in numerical order over a period of time, usually one mode per week, proceeding to the next higher number each Sunday and beginning with the 1st mode again when the 8th is completed. And a book (the ‘liturgical’ oktōēchos) in which the chant texts are grouped by mode in numerical order to facilitate performance according to the calendric oktōēchos (see Liturgy and liturgical books, §IV, 3, (viii)); books also exist in which chants are arranged according to mode but without regard to a calendar, notably the Western Tonary and the Byzantine Heirmologion).
Although many theories regarding the origins of the eight-mode system have been proposed, the earliest genuine evidence of the musical ...
(Lat.: ‘to be repeated’)
In Western chant, a section to be repeated, such as the refrain in hymns or the last part of the respond of a responsory, which is repeated after the psalm verse. In Ordo romanus I (second half of the 8th century) and later, the term ‘versus ad repetendum’ designated extra psalm verses added as needed to the Mass introit and communion. According to Husmann, the words ‘ad repetendum’ were also used in the Middle Ages for additional tropes to the introit antiphon....