Octoechos (liturgy)
The Octoechos book (from the Greek: ἡ [βίβλος] Ὀκτώηχος Ancient Greek pronunciation: [okˈtoixos]; from ὀκτώ 'eight' and ἦχος 'sound, mode' called echos; Church Slavonic: Осмѡгласникъ, Osmoglasnik from о́смь 'eight' and гласъ 'voice, sound') is a liturgical book containing a repertoire of hymns ordered in eight parts according to eight echoi (tones or modes). Originally created in the Monastery of Stoudios during the 9th century as a hymnal complete with musical notation, it is still used in many rites of Eastern Christianity.