Plato, Laws 812d

τούτων τοίνυν δεῖ χάριν τοῖς φθόγγοις τῆς λύρας προσχρῆσθαι, σαφηνείας ἕνεκα τῶν χορδῶν, τόν τε κιθαριστὴν καὶ τὸν παιδευόμενον, ἀποδιδόντας πρόσχορδα τὰ φθέγματα τοῖς φθέγμασι· τὴν δʼ ἑτεροφωνίαν καὶ ποικιλίαν τῆς λύρας, ἄλλα μὲν μέλη τῶν χορδῶν ἱεισῶν, ἄλλα δὲ τοῦ τὴν μελῳδίαν συνθέντος ποιητοῦ, καὶ δὴ καὶ πυκνότητα μανότητι καὶ τάχος βραδυτῆτι καὶ ὀξύτητα βαρύτητι σύμφωνον

So, to attain this object, both the lyre-master and his pupil must use the notes of the lyre, because of the distinctness of its strings, assigning to the notes of the song notes in tune with them; but as to divergence of sound and variety in the notes of the harp, when the strings sound the one tune and the composer of the melody another, or when there results a combination of low and high notes, of slow and quick time, of sharp and grave,