Of the words which have both pitches, some have the grave fused with the acute on the same syllable, and we call these “circumflexed”; others have them falling on separate syllables, and each retains its own quality. Now in words of two syllables there is no interval intermediate between low and high pitch; while in polysyllabic words, however long, only one syllable carries the acute accent among the many others in low pitch. ― translation from: Stephen Usher, opere citatoLCL 466 (1985), chapter xi, paragraph 4, page 79, lines 1–10
ante AD 210, Sextus Empiricus (author), Jürgen Mau (editor), Πρὸς Μαθηματικούς in Sexti Empirici Opera rec. Hermannus Mutschmann, Leipzig: in aedibus B.G. Teubneri, volume III: Adversus Mathematicos libros I–VI continens (1954), book i: «Πρὸς Γραμματικούς», chapter: ‹ὅτι ἀμέθοδόν ἐστι καὶ ἀσύστατον τὸ τεχνικὸν τῆς γραμματικῆς μέρος›, § 119 (page 31, lines 14–22; Bekk. p. 626, lines 11–20):