Fai: The Coptic Letter Taken from Demotic

Fai is the twenty seventh letter of the Coptic alphabet and the second of the seven taken from Demotic. It writes an f sound in native Egyptian words and carries no numeral value, standing outside the Greek alphabetic numeral series along with the other Egyptian additions. Coptic gives it constant grammatical employment: fai is the letter of the third person masculine singular pronoun in a range of constructions, so it appears throughout narrative and in verbal forms, and a student of Coptic meets it in the first weeks. Its situation is complicated by the fact that Coptic also has phi. The Greek phi had written an aspirated p in classical Greek and had become an f by the Roman period, and Coptic took over the later value, so the language ended up with two letters for what a modern ear hears as one sound.

In practice they are not interchangeable. Which letter a word takes depends on the word, and broadly on whether it belongs to the Egyptian layer of the vocabulary or the Greek one, so the choice is a piece of etymological information sitting in plain sight on the page. Duplications of this kind are common in borrowed alphabets and are usually a sign of exactly this history: a sound covered twice because two systems were merged.