Discussion of Greek genitive accentuation changes arising from synaeresis/syncope in straight (εὐθεῖα) contexts, with examples (φῶς→φωτός, ὠτός, φρήτος, υἷος from υἵϊος, Θῶνος from Θώνιος) and explanation of trisyllabic vs disyllabic forms.
ἐν τῇ εὐθείᾳ μόνον οἷον ὄϊς οἶς καὶ λοιπὸν ἀπὸ τῆς συνῃρημένης εὐθείας ἐκλίθη οἰός ἡ γενική. καὶ τούτου χάριν ὡς μὴ οὖσα ἡ γενικὴ τρισύλλαβος δυνάμει, ἀλλὰ κυρίως δισύλλαβος ὠξύνθη. τοῦτο δὲ καὶ ἐπὶ ἄλλων εὑρίσκεται, φημὶ δὲ τὸ ἐν τῇ εὐθείᾳ μόνον γίνεσθαι συναίρεσιν καὶ λοιπὸν ἐκ τῆς συνῃρημένης εὐθείας κλίνεσθαι τὴν γενικὴν ὡς ἐπὶ τοῦ φῶς φωτός καὶ ὦς ὠτός. ἔστι γὰρ φόας φόατος καὶ ὄας ὄατος καὶ λοιπὸν κρᾶσις παρακολουθεῖ τῇ εὐθείᾳ καὶ γίνεται φῶς καὶ ὦς κἀκεῖθεν κλίνεται φωτός καὶ ὠτός ἡ γενική. οὐδὲ γὰρ δυνάμεθα τὴν φωτός καὶ ὠτός γενικὴν ἀπὸ τῆς φόατος καὶ ὄατος λέγειν κατὰ κρᾶσιν, ἐπεὶ φῶτος καὶ ὦτος ὤφειλεν εἶναι βαρυτόνως ὡς ἀπὸ τρισυλλάβου γενικῆς γινόμενα. ἀλλ´ ὡς εἴρηται ἐν τῇ εὐθείᾳ ἐγένετο ἡ κρᾶσις κἀκεῖθεν ἡ κλίσις. σεσημείωται τὸ φρητός ὀξυνόμενον. ἀπὸ γὰρ τρισυλλάβου γενικῆς τῆς φρέατος γενόμενον κατὰ κρᾶσιν φρῆτος ὤφειλε βαρύνεσθαι. οὐδὲ γὰρ δυνάμεθα λέγειν ὅτι ἐπὶ τούτου ἐν τῇ εὐθείᾳ ἐγένετο ἡ κρᾶσις κἀκεῖθεν ἡ κλίσις. οὐδὲ γὰρ εἴρηται ἡ φρῆρ εὐθεῖα τῇ χρήσει. ἔστιν οὖν εἰπεῖν, ὅτι ἡ φρητός γενικὴ συνέδραμε κατὰ τὴν τάσιν τῇ Κρητός γενικῇ καὶ σητός καὶ θητός. —τὸ »υἷος ἑῆος« (Ο 138) ἐκ τρισυλλάβου τοῦ υἵϊος προπαροξυτόνου, οὗ ἡ εὐθεῖα υἷις. ἡ »Θῶνος παράκοιτις« (δ 228) σεσημείωται· εἰ γὰρ ἐκ τῆς »Θώνιος ἐν ψαμάθοις« (Nic. Ther. 312) συνεκόπη, κἂν ὠξύνετο ὡς ἡ πατρός μητρός· ἀλλ´ οὐδὲ συνῄρηται ἐκ τῆς Θόωνος, ὥς τινες, ἀμαρτύρου. οὐδαμοῦ γὰρ ἡ Θόων εὐθεῖα εὕρηται σημαίνουσα τὸν Αἰγύπτιον ἄνδρα, ὅντινα σημαίνει ἡ Θῶνος γενική, ἀλλ´ ἕτερον ἄνδρα ὡς παρ´ Ὁμήρῳ »Θόωνά τε Οἰνόμαόν τε« (Μ 140). ἔστιν οὖν εἰπεῖν, ὅτι κατὰ συγκοπὴν γέγονε ἡ Θῶνος γενικὴ ἀπὸ τῆς Θώνιος γενικῆς.
Only in the nominative, as for example ὄϊς, οἶς; and thereafter the genitive was declined from the contracted nominative, οἰός. And for this reason, since the genitive is not trisyllabic in potentiality but properly disyllabic, it was accented with an acute. This is found also in other cases: I mean that contraction occurs only in the nominative, and thereafter the genitive is declined from the contracted nominative, as in φῶς, φωτός, and ὦς, ὠτός. For there are φόας, φόατος, and ὄας, ὄατος, and thereafter crasis accompanies the nominative and φῶς and ὦς arise, and from there the genitive is declined as φωτός and ὠτός. For we are not able to say that the genitive φωτός and ὠτός is from φόατος and ὄατος by crasis, since it ought to be φῶτος and ὦτος with a grave accent, as being produced from a trisyllabic genitive. But, as has been said, the crasis occurred in the nominative, and from there the inflection.
φρητός is noted as being accented with an acute. For, being produced by crasis from the trisyllabic genitive φρέατος, it ought to be φρῆτος and to be accented with a grave. For we are not able to say that in this case the crasis occurred in the nominative and from there the inflection; for the nominative φρῆρ is not said in usage. It is possible, then, to say that the genitive φρητός has coincided in accent with the genitive Κρητός, and with σητός and θητός.
—The “υἷος ἑῆος” (Ο 138) is from the trisyllabic υἵϊος, proparoxytone, whose nominative is υἷις. The “Θῶνος παράκοιτις” (δ 228) is noted: for if it had been syncopated from “Θώνιος ἐν ψαμάθοις” (Nic. Ther. 312), it would also have been accented with an acute, like πατρός, μητρός; but neither has it been contracted from Θόωνος, as some say, without attestation. For nowhere is the nominative Θόων found meaning the Egyptian man whom the genitive Θῶνος means, but rather another man, as in Homer, “Θόωνά τε Οἰνόμαόν τε” (Μ 140). It is possible, then, to say that by syncope the genitive Θῶνος has come from the genitive Θώνιος.