Recently, I came across an excerpt from a scholium on Dionysius Thrax:
Διὰ τί τὸ "η" πρὸ τοῦ "τ" ψιλοῦται, ἐν δὲ τῷ ἧτα τῷ ὀνόματι τοῦ στοιχείου δασύνεται; Ἐπειδὴ παρὰ τοῖς ἀρχαίοις ὁ τύπος τοῦ "Η" ἐν τύπῳ δασείας ἔκειτο, ὥσπερ καὶ νῦν τοῖς Ῥωμαίοις.
Why is Ē before T pronounced smooth, when the name of the letter "hēta" is pronounced rough? It's because the ancients applied the shape of the hēta to the rough-breathing mark—like the Romans still do nowadays. (Trans. mine)
This is a nice little bit of trivia (that grammarians in the time of Dionysius Thrax still associated heta with
/h/). But that first bit makes me wonder.
Is it a rule that heta before tau is always smooth? If so, are there other rules like this that allow us to predict rough versus smooth breathing? If not, what did the author mean by this? (I unfortunately have no context for the quote, which might provide more explanation.)