Note 18 — § 20. Tonality of the Greek musical scale.
Reading edition
So in the Euclidean Sectio Canonis the propositions which deal with the 'movable' notes, viz. Paranêtê and Lichanos (Theor. xvii) and Parhypatê and Tritê (Theor. xviii), begin by postulating the Mesê (estô gar mesê ho B k.t.l.).