Octave species various available intervals could be combined in any order, even restricting species to just the diatonic genus would result in twelve ways of dividing the octave (and his 17th-century editor, Marcus Meibom, pointed out that the actual number is 21), but "only seven species or forms are melodic and symphonic" . Those octave species that cannot be mapped onto the system are therefore rejected . In chant theory beginning in the 9th century, the "New Exposition" of the composite treatise called "Alia musica" developed an eightfold modal system from the seven diatonic octave species of ancient Greek theory, transmitted to
Octave (liturgy) Epistle and Gospel of the Feast are repeated again at the Divine Liturgy. The term "octave" is applied to some church observances that are not strictly liturgical. For example, many churches observe an annual "Octave of Prayer for Christian Unity", which runs from 18 January to 25 January. The octave was established in 1895 by Pope Leo XIII for the period between Ascension and Pentecost. In 1909, Pope Pius X approved the transfer of this octave to the period between the former feast of the Chair of St. Peter and the feast of the Conversion of St. Paul. In 1968,