OKTOECHOS - (Octoechos, or Book of the Eight Tones) Service book containing the Canons and hymns of the eight Tones, used at Vespers and Matins services.
It contains poetic hymns, in the form of troparia, kontakia, canons, and so forth. They are divided into eight groups of melodies, or tones. Each tone contains the hymnody for an entire week, so that the complete Octoechos is repeated every eight weeks throughout most of the year. The arrangement of ecclesiastical chanting in tones was the work of the famous hymnographer of the Byzantine Church, St. John of Damascus (eighth century).