Octoechos

Oktōēchos (here transcribed "Octoechos"; Greek: ὁ Ὀκτώηχος [okˈtóixos]; from ὀκτώ "eight" and ἦχος "sound, mode" called echos; Slavonic: Осмогласие, Osmoglasie from о́смь "eight" and гласъ, Glagolitic: ⰳⰾⰰⱄⱏ, "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.