Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Nana holds the status one of the two "special" additional echoi in the system of the Byzantine Octoechos. The other one is called nenano. Nana and nenano have been characterized in Byzantine theoretical treatises as both echoi and "not echoi, but phthorai". Phthorai are modulation signs and/or alteration signs. Theoreticians of various mostly yet unpublished treatises as for example codex EBE 899 of the National Greek Library in Athens, try to explain the difference between echos (mode), modulation, and alteration of diatonic degree, but these concepts are very difficult to explain without recourse either to a staff system notation, to a mathematical framework of ratios of frequencies of pitches, or to an experimental framework of lengths of sounding strings or pipes. Thus, the distinction between mode, modulation and alteration remains somewhat unsharp in the notational system.

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