Echos 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, differentiated according to the chant genre and according to the performance style. It is akin to a Western medieval tonus, an Andalusian tab', an Arab naġam, or a Persian parde.

Phthora nana is one of the ten modes of the Hagiopolitan Octoechos consisting of 8 diatonic echoi and two additional phthorai. It is used in different traditions of Orthodox chant until today. The name "nana" is taken from the syllables sung during the intonation which precedes a melody composed in this mode. The name "phthora" derived from the verb φθείρω and means "destroy" or "corrupt". It was usually referred to the diatonic genus of the eight mode system and as a sign used in Byzantine chant notation it indicated a "change to another genus", in the particular case of phthora nana a change to the enharmonic genus. Today the "nana" intonation has become the standard name of the third authentic mode which is called "echos tritos" in Greek and "third glas" in Old Church Slavonic.