Coptic was never a single written standard. Several regional dialects were committed to writing, of which Sahidic, centred on the Nile valley of Upper Egypt, became the dominant literary form in the first Christian millennium, while Bohairic, of the Delta, later replaced it as the language of the Coptic Orthodox Church and remains its liturgical language today. Alpha appears in every one of them, but not always in the same places, because the dialects differ in precisely the vowels that the Greek alphabet had at last made visible. Words written with alpha in one dialect appear with a different vowel letter in another, and comparative tables of those correspondences are a standard tool for placing an undated manuscript. This is a capacity the older Egyptian scripts simply did not have: a Demotic text conceals the vowel differences that a Coptic text displays, so the linguistic history of Egyptian before Coptic has to be reconstructed indirectly.
Alpha also sits at the centre of Coptic scribal habit. The letter is written with a single flowing movement in the round uncial hands of the great biblical codices, and in the more angular later hands it becomes a compact triangle. Scribes marked it with the superlinear stroke that Coptic uses to show a syllabic consonant nearby, and they used it as a numeral in the margins. In the Nag Hammadi codices, a Coptic library of the fourth century containing translations of Greek religious and philosophical works, alpha stands both for the Egyptian vowel and for the Greek one, in the same line, in the same hand.