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).

The noun echos in Greek means "sound" in general. It acquired the specialized meaning of mode early on in the development of Byzantine music theory since the Octoechos reform in 692.

In general, the concept of echos denotes a certain octave species, its intervallic structure as well as a set of more or less explicitly formulated melodic rules and formulae that represent a certain category of melodies within the musical genre. As such, echos is the basis for composing or improvising new melodies that belong to it, as well as for properly performing existing pieces that have been written in it. These rules include the distinction of a hierarchy of degrees (tones, notes), where certain degrees figure as cadence notes (ἑστώτες) around which the melody will revolve prominently, or on which the melody will end most of the time. However, only very late stages of the theory (19th-20th century) actually provide systematic descriptions of echoi, while earlier stages use mostly diagrams, indirect descriptions and examples. Explicit detailed descriptions must still be provided based on extensive analysis, as is the case with modal phenomena in numerous other cultures.

Early treatises only state the initial or "base" degree (η βασή) which is the tone sung as a burden (ἴσον) by certain singers of the choir called isokrates in order to support any melody composed in a certain echos. By this support singers (psaltes) could easily recognise the relative position of each note as it was organised by tetrachords based on the basis note of each echos. This base degree of the mode was communicated by an intonation formula of a foresinger, known as enechema.

There are different styles by which to notate enechema which are crucial to the understanding of the different chant books and their notation. All these forms were written in red ink. The explicit long form was called by Jørgen Raasted intonation, but only the books of the cathedral rite used such explicit intonations, also between the sections, where these intonations were called medial intonation. This explicit form made sense, since the intonation also communicated the changes between the left and the right choir and their leaders performed these intonations to coordinate these changes. There was a short form as well which was called modal signature. It indicated the echos by the numeral like πλα' for "plagios protos," while the neumes sung with the last syllable of the enechema were written above right to the numeral. This short form was used in two different ways, as main signature (in the table called αἱ μαρτυρίαι τῶν ἤχων, "the witnesses of the echoi") it indicated the echos of a whole composition, but especially in sticheraria notators also wrote medial signatures (in the table called αἱ μαρτυρίαι τῶν φθόγγων, "the witnesses of phthongoi", pitches memorised by echemata) between the neumes above a kolon of the text, in order to indicate that the melos changed here into another echos. The traditional Greek term for these medial signatures was "martyria" (μαρτυρία), since the medial signature also "testified" the phthongos of the cadence made at the kolon.

Within the dialogue treatise (erotapokriseis) a catalogue of short formulas memorizes each echos of the Hagiopolitan octoechos and its two phthorai (νενανῶ and νανὰ). These formulas are also called "echemata" (ἠχήματα)—or more often "enechemata" (ἐνηχήματα) or "apechemata" (ἀπηχήματα). The use echemata was also imitated by Carolingian cantors who used similar intonation formulas and collected them in a separate book called tonary.

Περὶ πλαγίων

Ἀπο τοῦ πλαγίου πρώτου ἤχου πάλιν καταβαίνεις τέσσαρας φωνάς, καὶ εὑρίσκεται πάλιν πλάγιος πρώτου· ὅυτως δὲ / ἄνανε ἄνες ἀνὲ ἄνες·