Accentuation rules for Greek words ending in -ιος, showing proparoxytone/paroxytone patterns with examples (ethnic/place names, adjectives, adverbs).
Τὰ διὰ τοῦ 'ιος' τρισύλλαβα εἰ ἔχοιεν 'υ' ἐν τῇ πρώτῃ συλλαβῇ, προπαροξύνεται, εἰ μή τις διαστολὴ εἴη ἢ κύριον ὑπάρχοι τρίβραχυ, κύριος, Κύπριος, Τύριος, μύλιος καὶ Μύλιος ἔθνος Φρυγίας. Ἑκαταῖος Ἀσίᾳ. Πύλιος, Φρύγιος, Κύρνιος, Λύκιος, Λύγκιος, Λύττιος, Λύκτιος, Πύξιος, Μύνδιος, Πύρνιος, Ξύλιος, Ῥύβδιος, Σύριος, νύχιος, Στύγιος, Μύσιος, Πύθιος, βύθιος, δύϊος, κύκλιος. τὸ Τυχίος, Φρυγίος, Πλυτίος, Κλυτίος, Σκυφίος παροξύνεται ὡς κύρια. μύριος ὁ ὡρισμένος ἀριθμός, μυρίος ἀόριστος. ἑκάτερα δὲ καὶ ὁ μύριος καὶ τὰ μυρία ὤφειλεν ὀξυτονεῖσθαι κατὰ τὴν τρίτην ἀπὸ τέλους. τὸ γὰρ ἀρσενικὸν προπαροξύνεται κατὰ τὸ Λύδιος καὶ τὸ μύριον δὲ ὁμοίως, ἐπεὶ τὰ παρεσχηματισμένα οὐδέτερα ἀρσενικοῖς ὁμότονα αὐτοῖς ἐστιν. καὶ νύμφιος οἶκος προπαροξύτονον, νυμφίος δὲ ὁ γαμετής παροξύτονον. Ὅσα ἀπὸ ἐπιρρημάτων παρῆκται διὰ τοῦ 'ιος', προπαροξύνεται, πρωΐ πρώϊος, ὀψέ ὄψιος, νόσφιν νοσφίδιος, ἶφι ἴφιος, ἀεί ἀΐδιος, ῥέα ῥηΐδιος, ἐκτάδην ἐκτάδιος, ἀμφαδόν ἀμφάδιος, ἐνθάδε ἐνθάδιος, σταδόν στάδιος, ὀπίσθιος, μαψίδιος. σεσημείωται ἄντην ἀντίος, σχεδόν Σχεδίος ὡς κύριον. * Τὰ διὰ τοῦ 'ιος' τριβράχεα ἐπὶ κυρίων παροξύνεται οἷον Σχεδίος, Χρομίος, Κλυτίος, Ὁδίος, Δολίος, Τυχίος. ἀντιπίπτουσι δὲ κατὰ παράδοσιν τὸ Ἅλιος, Ἄνιος, Ξένιος, Κρόνιος, Ὕπιος ποταμὸς καὶ πόλις ὑπὸ τὴν Ποντικὴν Ἡράκλειαν, ὡς Δομίτιος Καλλίστρατος ἐν ἕκτῳ τῶν περὶ Ἡρακλείας. λέγεται καὶ ἀρσενικῶς καὶ θηλυκῶς. γίνεται δὲ παρὰ τὴν ὑπό πρόθεσιν ὡς Ἄνιος παρὰ τὴν ἀνά καὶ Ἄμφιος ὁ τοῦ Σελάγου υἱὸς παρὰ τὴν ἀμφί ἐκτάσεως γενομένης τοῦ 'ι' ποιητικῆς. Τὰ εἰς 'ιος' τρισύλλαβα κύρια ἔχοντα τὴν τρίτην μακρὰν προπαροξύνεται,
Trisyllables in -ιος, if they have υ in the first syllable, are accented on the antepenult, unless there is some distinction or they are a proper name that is tribrach: κύριος, Κύπριος, Τύριος, μύλιος, and Μύλιος, a people of Phrygia; Ἑκαταῖος in Asia; Πύλιος, Φρύγιος, Κύρνιος, Λύκιος, Λύγκιος, Λύττιος, Λύκτιος, Πύξιος, Μύνδιος, Πύρνιος, Ξύλιος, Ῥύβδιος, Σύριος, νύχιος, Στύγιος, Μύσιος, Πύθιος, βύθιος, δύϊος, κύκλιος. But Τυχίος, Φρυγίος, Πλυτίος, Κλυτίος, Σκυφίος are accented on the penult as proper names. μύριος is the definite number, μυρίος the indefinite. Yet both μύριος and τὰ μυρία ought to be oxytone on the third syllable from the end; for the masculine is accented on the antepenult like Λύδιος, and μύριον likewise, since the derived neuters are not of the same accent as the masculines themselves. And νύμφιος οἶκος is proparoxytone, but νυμφίος, the bridegroom, is paroxytone. Whatever is derived from adverbs in -ιος is accented on the antepenult: πρωΐ → πρώϊος, ὀψέ → ὄψιος, νόσφιν → νοσφίδιος, ἶφι → ἴφιος, ἀεί → ἀΐδιος, ῥέα → ῥηΐδιος, ἐκτάδην → ἐκτάδιος, ἀμφαδόν → ἀμφάδιος, ἐνθάδε → ἐνθάδιος, σταδόν → στάδιος, ὀπίσθιος, μαψίδιος. Noted as exceptions are ἄντην → ἀντίος, and σχεδόν → Σχεδίος as a proper name. Tribrachs in -ιος, when they are proper names, are accented on the penult, e.g. Σχεδίος, Χρομίος, Κλυτίος, Ὁδίος, Δολίος, Τυχίος. But by tradition the following are exceptions: Ἅλιος, Ἄνιος, Ξένιος, Κρόνιος, Ὕπιος, a river and a city near Pontic Heraclea, as Domitius Callistratus says in the sixth book of his work On Heraclea. It is said both in the masculine and in the feminine. And it is formed from the preposition ὑπό, just as Ἄνιος is from ἀνά, and Ἄμφιος, the son of Selagus, from ἀμφί, with a lengthening of ι in poetic usage. Trisyllabic proper names in -ιος having the third syllable long are accented on the antepenult,