The Octoechos
The Octoechos (from the Greek: ἡ Ὀκτώηχος Greek pronunciation: [okˈtoixos]; from ὀκτώ «eight» and ἦχος «sound, mode» called echos; 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. The book with similar function in the Western Church is the tonary, and both contain the melodic models of an octoechos system; however, while the tonary serves simply for a modal classification, the octoechos is organized as a cycle of eight weeks of services. The word itself can also refer to the repertoire of hymns sung during the celebrations of the Sunday Office.