Octoechos

Oktōēchos (here transcribed "Octoechos"; Greek: Ὀκτώηχος, from ὀκτώ "eight" + ἦχος "sound, mode" called echos; Slavonic: Октоихъ, Oktoikh, or Осмогласникъ, Osmoglasnik from о́семь "eight" and гласъ "voice") is the name of the eight mode system used for the composition of religious chant in Byzantine, Syrian, Coptic, Armenian, 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 Orthodox chant today.