Discussion of the formation and accentuation of the participles ἑκών and ἀέκων, reasons for their tonic changes and classification as participles rather than nouns; followed by rules about accent/tone of articles.
ὧν πρώτη ἐστὶν αὕτη. αἱ μετοχαὶ ἢ ἁπλαῖ εἰσιν ἢ παρασύνθετοι, οὐδέποτε δὲ σύνθετοι· ἐπειδὴ οὖν ἡ ἀέκων σύνθετός ἐστιν, οὐδὲ γὰρ ἔχει ῥῆμα προϋπάρχον ἐν συνθέσει, ἵνα παρασύνθετον αὐτὴν εἴπωμεν, ἀλλ´ ἐν τῇ μετοχῇ ἐγένετο ἡ σύνθεσις οἷον ἑκών ἀέκων, τούτου χάριν ἐν τῇ συνθέσει ἀνεβίβασε τὸν τόνον, ἵνα δείξῃ, ὅτι σύνθετός ἐστι καὶ οὐ παρασύνθετος ὡς ἄλλαι μετοχαί. δευτέρα δὲ αἰτία ἐστὶν αὕτη· ἐκ τοῦ εἴκω τὸ ὑποχωρῶ γέγονε ἡ ἑκών κατὰ πάθος τουτέστι κατὰ ἀποβολὴν τοῦ 'ι', καὶ ἐπειδὴ τὸ εἴκω ῥῆμα συνέστειλε τὴν παραλήγουσαν ἐν τῇ ἑκών μετοχῇ, τούτου χάριν ὠξύνθη ὁμοίως ταῖς ἄλλαις μετοχαῖς ταῖς βραχείᾳ παραληγομέναις, φημὶ δὴ ταῖς τοῦ δευτέρου ἀορίστου, ἀλλ´ ἐπειδὴ ἡ ἑκών οὐκ ἔστι δευτέρου ἀορίστου, ἀλλ´ ἐνεστῶτος καὶ παρατατικοῦ, δηλονότι οὐκ ὤφειλεν ὀξύνεσθαι ὁμοίως ἐκείναις. Ἐπειδὴ οὖν παραλόγως συνέδραμεν ἐκείναις καὶ ὠξύνθη ἡ ἑκών μετοχή, τούτου χάριν ἐν τῇ συνθέσει, φημὶ δὴ ἐν τῇ ἀέκων ἐβαρύνθη, ἵνα τὴν παράλογον τάσιν τοῦ ἁπλοῦ ἐκφύγῃ, φημὶ δὴ τὴν ὀξεῖαν. —ἄξιον δέ ἐστι ζητῆσαι, διὰ τί ἡ ἑκών μετοχὴ ἐν τῇ ἀέκων ἀναβιβάσασα τὸν τόνον ἔμεινε μετοχὴ καὶ οὐκ ἐγένετο ὄνομα; καὶ ἔστιν εἰπεῖν, ὅτι λέξις ἐκ τῆς 'α' στερήσεως συντιθεμένη καὶ φυλάττουσα τὸ τέλος, φυλάττει καὶ τὸ αὐτὸ μέρος τοῦ λόγου οἷον δοῦλος ἄδουλος, φίλος ἄφιλος, σεμνός ἄσεμνος, κακός ἄκακος, ψευδής ἀψευδής, σαφής ἀσαφής· τὸ γὰρ τλάς Ἄτλας καὶ τὸ βάς Ἄβας φυλάξαντα τὸ τέλος καὶ μὴ φυλάξαντα τὸ αὐτὸ μέρος τοῦ λόγου, ἀλλὰ γενόμενα ὀνόματα οὐκ ἀντίκειται ἡμῖν, ἐπειδὴ οὐκ ἔχουσι τὴν 'α' στέρησιν, ἀλλὰ τὴν 'α' ἐπίτασιν οἱονεὶ ὁ πάνυ καρτερῶν, πάνυ βαίνων· ἐπειδὴ οὖν ἡ ἀέκων ἐφύλαξε τὸ τέλος τῆς ἑκών, τούτου χάριν καὶ τὸ αὐτὸ μέρος τοῦ λόγου ἐφύλαξε καὶ ἔμεινε μετοχή. {1ΠΕΡΙ ΤΟΝΟΥ ΑΡΘΡΩΝ ΑΝΤΩΝΥΜΙΩΝ ΚΑΙ ΠΡΟΘΕΣΕΩΝ. ΒΙΒΛΙΟΝ ιηʹ. Περὶ ἄρθρων.}1 Πᾶν ἄρθρον ὀξύνεται, χωρὶς τῶν γενικῶν καὶ δοτικῶν· αὗται γὰρ περισπῶνται, τοῦ τῷ, τῆς τῇ, τοῖν ταῖν, τῶν, τοῖς ταῖς. καὶ τὰ τούτων δὲ ὑποτακτικά, ἀποβολῇ τοῦ 'τ' γινόμενα, περισπῶνται. ὁμοίως καὶ τὸ κλητικὸν ὦ ἄρθρον μὲν οὐκ ἔστιν, ἀλλ´ ἐπίρρημα, περισπᾶται δὲ ὅμως καὶ ψιλοῦται τῶν ἀπὸ φωνηέντων ἀρχομένων ἄρθρων δασυνομένων
Of these the first is this. Participles are either simple or parasynthetic, but never compound. Since, then, ἀέκων is compound—for it does not even have a verb pre-existing in composition, so that we might call it parasynthetic, but the composition arose in the participle itself, as in ἑκών, ἀέκων—for this reason in the compound it raised the accent, in order to show that it is compound and not parasynthetic like other participles. A second cause is this: from εἴκω, “I yield,” ἑκών has arisen by affection, that is, by loss of the ‘ι’; and since the verb εἴκω contracted the penult in the participle ἑκών, for this reason it was accented with an acute, like the other participles with a short penult—I mean those of the second aorist. But since ἑκών is not of the second aorist but of the present and imperfect, plainly it ought not to be accented with an acute like those. Since, then, ἑκών the participle ran together with those contrary to rule and was accented with an acute, for this reason in the compound—I mean in ἀέκων—it was given a grave, so as to escape the anomalous tension of the simple form, I mean the acute.
It is worth inquiring why the participle ἑκών, having raised the accent in ἀέκων, remained a participle and did not become a noun. And one may say that a word compounded with the privative α and preserving the ending preserves also the same part of speech, as δοῦλος, ἄδουλος; φίλος, ἄφιλος; σεμνός, ἄσεμνος; κακός, ἄκακος; ψευδής, ἀψευδής; σαφής, ἀσαφής. For τλάς, Ἄτλας and βάς, Ἄβας, which preserve the ending but do not preserve the same part of speech, having become nouns, do not stand in our way, since they do not have the privative α but the intensive α, as it were “one who is very enduring,” “one who goes very much.” Since, then, ἀέκων preserved the ending of ἑκών, for this reason it also preserved the same part of speech and remained a participle.
{ON THE ACCENT OF ARTICLES, PRONOUNS, AND PREPOSITIONS. BOOK 18. On articles.} Every article is accented with an acute, except the genitives and datives; for these are circumflexed: τῷ, τῆς, τῇ, τοῖν, ταῖν, τῶν, τοῖς, ταῖς. And the subordinated forms of these too, produced by dropping the ‘τ’, are circumflexed. Likewise the vocative ὦ, though it is not an article but an adverb, is nevertheless circumflexed, and it is psilotic, while the articles beginning with vowels are aspirated.