Ancient Greek Music: A Technical History by Stefan Hagel
... highest: e, f, g, a (mesē diezeugmenōn), b, c, d, e’. But in order to
notate the conjunct upper tetrachord of the Lower Perfect System,
we will need one accidental, namely b♭: e, f, g, a (mesē synnēmenōn),
b♭, c, d [see Figure 2]. This will be like modulating to a key with
one flat from a natural key ...