About the Mystery of the Letters the mid-6th century, in which Mar Saba had played an important role. The anonymous author of the treatise declares he was prompted to the study of the secret meanings of the letters by the words of the Apocalypse, "I am the Alpha and Omega", and that he subsequently received a vision about them on Mount Sinai. The author proposes a re-modelled Greek alphabet reduced to 22 letters on the model of the 22 letters of Hebrew (discounting Xi Ξ and Psi Ψ), and with the Hebrew letter names. He then interprets these letters as symbolic figures of 22 works of
Biblical Hebrew phonemes. After a sound shift the letters , could only mark one phoneme, but (except in Samaritan Hebrew) still marked two. The old Babylonian vocalization system wrote a superscript above the to indicate it took the value , while the Masoretes added the shin dot to distinguish between the two varieties of the letter. The original Hebrew alphabet consisted only of consonants, but gradually the letters , , , , also became used to indicate vowels, known as "matres lectionis" when used in this function. It is thought that this was a product of phonetic development: for instance, *bayt ('house')