An echos (Ancient Greek: lang = "grc"> ἦχος , echos, "sound", ancient Greek pl .: ἦχοι , echoi) is, in the theory of Byzantine music, that of the eight modes of the octoechos system in which a composition is to be sung. Each echo controls various types of rhythms, metrics, songs, and performance styles. It is similar to the plainchant mode, the Andalusian tab, the Arabic maqam or the Persian dastgâh.
The term echos acquired the sense of fashion very early in the development of the Byzantine music since a reform of the octoechos at the Council in Trullo of 692.
Basically, the term echos refers to the musical scale, that is the arrangement of the intervals of notes, but also a more or less explicit set of rules and melodic formulas, thus defining categories of melodies. The melodies are thus arranged or composed for a given echo; echoes also control the style of interpretation. These rules control a hierarchy of degrees (tone, notes); certain degrees form the cadences (ἑστώτες), notes around which the melody is solved mainly where the melody usually ends.
It is only very late, since XIX and XX centuries, that a systematic description of echos, whereas previously only diagrams, pictorial descriptions and examples were used. A detailed theory of the echo system is yet to be established, as is the case for the modal theories of music of many other cultures.