Discussion of accentuation rules for dual genitives ending in -αιν and -οιν, with examples (Ἀτρεΐδαιν, Ἀτρειδῶν, φίλαι/φίλαιν, etc.) and conditions depending on stem vowels and accent position (perispomenon, oxytone, barytone).
παροξύνεται. ὀξύνεται μὲν ἀπὸ περισπωμένων καὶ ὀξυνομένων χρυσοῦς χρυσώ, καλός καλώ. πρὸ μιᾶς δὲ ἔχει τὸν τόνον τὰ ἀπὸ βαρυνομένων, Ὅμηρος Ὁμήρω, φίλος φίλω. τὸ δὲ ἄμφω καὶ δύο θεματικά εἰσιν. {1Περὶ τόνου τῆς δυϊκῆς γενικῆς.}1 Αἱ γενικαὶ τῶν δυϊκῶν εἰς 'αιν' λήγουσι καὶ εἰς 'οιν'. αἱ μὲν οὖν εἰς 'αιν' ἐκ τῶν ἰδίων εὐθειῶν κανονίζονται. εἰ μὲν γὰρ ὀξύνονται αἱ εὐθεῖαι καὶ περισπῶνται, πάντως αἱ γενικαὶ περισπῶνται, καλά καλαῖν, ἀλλὰ καὶ διπλᾶ διπλαῖν. εἰ δὲ βαρύνονται, καὶ αἱ γενικαί, φίλαι φίλαιν, μοῦσαι μούσαιν. καὶ ἀποροῦσί τινες λέγοντες, διὰ τί ἡ Ἀτρείδαιν γενικὴ τῶν δυϊκῶν οὐ περισπᾶται ὁμοίως τῇ Ἀτρειδῶν γενικῇ τῶν πληθυντικῶν; ἀλλ´ ἔστιν εἰπεῖν, ὅτι ἡ Ἀτρείδαιν γενικὴ τῶν δυϊκῶν μᾶλλον τῇ Ἀτρείδαις δοτικῇ τῶν πληθυντικῶν ἀκολουθεῖ ἤπερ τῇ Ἀτρειδῶν γενικῇ τῶν πληθυντικῶν. κατὰ γὰρ τὴν φωνὴν τῇ δοτικῇ τῶν πληθυντικῶν πλησιάζει· τὴν αὐτὴν γὰρ δίφθογγον ἔχει. Ἄλλως τε δὲ πᾶσαι αἱ πτώσεις βαρύνονται οἷον Ἀτρείδης Ἀτρείδου Ἀτρείδῃ Ἀτρείδην, δυϊκὰ Ἀτρείδα Ἀτρείδαιν, πληθυντικὰ Ἀτρείδαις Ἀτρείδας. οὕτως οὖν καὶ Ἀτρείδαιν βαρυτόνως. Μόνη δὲ ἡ Ἀτρειδῶν γενικὴ τῶν πληθυντικῶν περιεσπάσθη, ἐπεὶ καὶ μόνη διῃρέθη οἷον Ἀτρειδέων. Ἐὰν βούλωνται οὖν περισπᾶν τὴν Ἀτρείδαιν γενικὴν τῶν δυϊκῶν ὁμοίως τῇ Ἀτρειδῶν γενικῇ τῶν πληθυντικῶν, καὶ μὴ βαρύνειν ὁμοίως ταῖς ἄλλαις πτώσεσι, δότωσαν ἐπ´ αὐτῆς τὸ ἐξαίρετον, ὅπερ ἔσχεν ἡ γενικὴ τῶν πληθυντικῶν, τουτέστι τὴν διαίρεσιν· εἰ δὲ οὐ διαιρεῖται, ἄρα οὐδὲ τὸν τόνον ἕξει τῆς γενικῆς τῶν πληθυντικῶν. Αἱ δὲ εἰς 'οιν' γενικαὶ εἰ μὲν ἀπὸ τῶν εἰς 'ω' γίνοιντο εὐθειῶν, βαρυνομένων μὲν συμβαρύνονται, φίλω φίλοιν, ὀξυνομένων δὲ περισπῶνται, καλώ καλοῖν. τὸ ἀμφοῖν καὶ δυοῖν καὶ δυεῖν, ὥς φασιν, κατὰ τροπὴν τοῦ 'ο' εἰς 'ε', οὐκ ἀκόλουθα τῷ ἄμφω καὶ δύο βαρυνομένοις. Εἰ δὲ ἀπὸ τῶν εἰς 'ε', ἀπὸ μὲν τῶν δισυλλάβων περισπῶνται, χεῖρε χειροῖν, θῆρε θηροῖν, πόδε ποδοῖν, φῶτε φωτοῖν, ἐπὶ ἀνδρῶν, φῶτε δὲ φώτοιν ἐπὶ πυρσῶν. πάλιν σεσημειωτέον τὸ τίνοιν πύσμα βαρυνόμενον ὡς πρὸς τὸ ἀόριστον περισπώμενον καὶ ἔτι τὸ παίδοιν, Τρώοιν, πάντοιν καὶ τὰ λοιπά, ἃ καὶ κατὰ τὴν γενικὴν
It is accented with an acute. It receives an acute from circumflexed and acute-accented forms: χρυσοῦς → χρυσώ, καλός → καλώ. But the forms derived from barytone words have the accent on the penult: Ὅμηρος → Ὁμήρω, φίλος → φίλω. But ἄμφω and δύο are thematic.
{1On the accent of the dual genitive.}1 The genitives of the dual end in -αιν and in -οιν. Those in -αιν are regulated from their own nominatives. For if the nominatives are acute-accented or circumflexed, the genitives are in every case circumflexed: καλά → καλαῖν, and likewise διπλᾶ → διπλαῖν. But if they are barytone, the genitives too are barytone: φίλαι → φίλαιν, μοῦσαι → μούσαιν. And some are at a loss, saying: why is the dual genitive Ἀτρείδαιν not circumflexed like the plural genitive Ἀτρειδῶν? But one may say that the dual genitive Ἀτρείδαιν follows rather the plural dative Ἀτρείδαις than the plural genitive Ἀτρειδῶν. For in sound it is close to the plural dative, since it has the same diphthong. Besides, all the cases are barytone, e.g. Ἀτρείδης, Ἀτρείδου, Ἀτρείδῃ, Ἀτρείδην; in the dual Ἀτρείδα, Ἀτρείδαιν; in the plural Ἀτρείδαις, Ἀτρείδας. Thus Ἀτρείδαιν too is barytone. Only the plural genitive Ἀτρειδῶν has been circumflexed, since it alone also underwent diaeresis, e.g. Ἀτρειδέων. If, then, they wish to circumflex the dual genitive Ἀτρείδαιν like the plural genitive Ἀτρειδῶν, and not to make it barytone like the other cases, let them grant it the exceptional feature which the plural genitive had—namely, diaeresis; but if it is not divided, then it will not have the accent of the plural genitive either.
The genitives in -οιν, if they are formed from nominatives in -ω, are barytone along with barytone bases: φίλω → φίλοιν; but from acute-accented bases they are circumflexed: καλώ → καλοῖν. ἀμφοῖν and δυοῖν and δυεῖν, as they say, by change of ο to ε, do not follow barytone ἄμφω and δύο. But if they are formed from nominatives in -ε, then from disyllables they are circumflexed: χεῖρε → χειροῖν, θῆρε → θηροῖν, πόδε → ποδοῖν, φῶτε → φωτοῖν (of men), but φῶτε → φώτοιν of torches. Again one must note τίνοιν, an interrogative, barytone as compared with the indefinite, which is circumflexed; and further παίδοιν, Τρώοιν, πάντοιν, and the rest, which also according to the genitive…