Echos (Greek: ἦχος [ˈixos] "sound", pl. echoi ἦχοι [ˈiçi]; Old Church Slavonic: гласъ [glasŭ] "voice, sound") is the name in Byzantine music theory for a mode within the eight-mode system (oktoechos), each of them ruling several melody types, and it is used in the melodic and rhythmic composition of Byzantine chant ("thesis of the melos"), differentiated according to the chant genre and according to the performance style ("method of the thesis"). It is akin to a Western medieval tonus, an Andalusian tab', an Arab naġam (since 1400 "maqam"), or a Persian parde (since 18th-century dastgah).