What is "octoechos"

Oktōēchos (here transcribed "Octoechos"; Greek: ; from ὀκτώ "eight" and ἦχος "sound, mode" called echos; Slavonic: Осмогласие, Osmoglasie from о́смь "eight" and гласъ, Glagolitic: , "voice, sound") is the name of the eight-mode system used for the composition of religious chant in Byzantine, Syriac, Armenian, Georgian, Latin and Slavic churches since the Middle Ages. In a modified form the octoechos is still regarded as the foundation of the tradition of monodic chant in the Byzantine Rite today.

The liturgical book called Octoechos (from the Greek: ; from ὀκτώ "eight" and ἦχος "sound, mode" called echos; Slavonic: Осмѡгласникъ, Osmoglasnik from о́смь "eight" and гласъ "voice, sound") contains a repertoire of hymns ordered in eight parts according to the eight echoi ( tones or modes). Originally created as a hymn book with musical notation in the Stoudios monastery during the 9th century, it is still used in many rites of Eastern Christianity. The hymn book has something in common with the book tonary of the Western Church. Both contained the melodic models of the octoechos system, but the tonary served simply for a modal classification, while the book octoechos is as well organized as a certain temporal of several eight week periods and the word itself means the repertoire of hymns sung during the celebrations of the Sunday Office.