Building upon the evidence discussed in Lynch 2022a and 2022b, this article offers the first account of the historical evolution and use of the Greek notation keys (tónoi) that bridges the gap between the Hellenistic and Imperial harmonic systems. More specifically, this article sheds light on the metamorphosis that turned the Classical Dorian-based harmonic system into the ‘Lydian’ system employed in Imperial Greek scores. In contrast with previous scholarship, the new picture offered in this article reconciles key theoretical insights provided by Ptolemy, Porphyry and other theoretical sources with documentary evidence that illustrates the structure of the Imperial harmonic system and its use in the Imperial musical documents (dDAGM). The new solution offered in this article entails a shift of a mere semitone between the tonal centre of the Classical harmonic system (DorianmésēF3) and its counterpart in the Imperial harmonic system (HypolydianmésēE3, which corresponded to the Classical modeLydistí). §§2–3 of the article show how this small shift in pitch had wide-ranging theoretical implications that affected the organisation of the Imperial harmonic system as a whole, as it undermined the central role that fourths had in the Classical system and introduced the new system described by Porphyry, which is based on a structural fifth. §4 shows how this new solution allows us to reconstruct, for the first time, a continuous, if evolving, tradition that stretches from Euripides’Orestesto late antiquity. SELECTBIBLIOGRAPHY Barker, A. (1989).Greek Musical Writings 2.Cambridge: CUP. Barker, A. (2007).The Science of Harmonics in Classical Greece.Cambridge: CUP. Barker, A. (2020). ‘Harmonics’ in Lynch, T. A.C. and Rocconi, E. (eds)A Companion to ancient Greek and Roman Music, Malden: Blackwell, 257–274. DAGM =Pöhlmann, E. and West, M.L. (2001).Documents of Ancient Greek Music: The Extant Melodies and Fragments. Oxford. dDAGM =Lynch, T.A.C. (2021).Database ‘Documents of Ancient Greek Music’.https://doi.org/10.5281/zenodo.5181743 Hagel, S. (2010).Ancient Greek Music – A New Technical History.Cambridge Lynch, T. (2018). ‘ “Without Timotheus, much of ourmelopoiiawould not exist; but without Phrynis, there wouldn’t have been Timotheus”: Pherecrates’ twelve strings, thestrobilosand the harmonicparanomiaof the New Music’,Greek and Roman Musical Studies6.2, 290–327. Lynch, T. A.C. (2020). ‘Tuning the Lyre, Tuning the Soul:Harmoníaand thekośmosof the Soul in Plato’sRepublicandTimaeus’,Greek and Roman Musical Studies8.1, 111–55. Lynch, T . A.C. (2022a). ‘Unlocking the Riddles of Classical Greek Melodies I: Dorian Keys to the Harmonic Revolution of the New Music and the Hellenistic Musical Documents’Greek and Roman Musical Studies10.2 [preprint:https://doi.org/10.5281/zenodo.5266960] Lynch, T. A.C. (2022b). ‘Unlocking the Riddles of Classical Greek Melodies II: the Revolution of the New Music in the Ashmolean Papyri (DAGM5–6) and Athenaeus’Paean(DAGM20)’,Greek and Roman Musical Studies10.2. [preprint:https://doi.org/10.5281/zenodo.5267084]. Martinelli, M.C. (2020). ‘Documenting Music’, in Lynch, T. A.C. and Rocconi, E. (eds)A Companion to ancient Greek and Roman Music, Malden: Blackwell, 103–115. West, M.L. (1994).Ancient Greek Music. Oxford: OUP.