History of the Greek alphabet retained the sound. A symbol based on the left-hand half ( ├ ) of the letter Η was therefore sometimes used to indicate the presence of where necessary, and its absence was indicated by a symbol based on the right half. During the classical period, ΕΙ came to be pronounced and ΟΥ came to be pronounced , Υ having meanwhile moved to . By about 200 BC, a system of diacritical marks was invented, representing the tone accents in use in Ancient Greek. This also helped to indicate the length of the vowels Α, Ι, and Υ in certain cases
Greek alphabet an additional diacritic in certain words, the so-called iota subscript, which has the shape of a small vertical stroke or a miniature below the letter. This iota represents the former offglide of what were originally long diphthongs, (i.e. ), which became monophthongized during antiquity. Another diacritic used in Greek is the diaeresis (), indicating a hiatus. This system of diacritics was first developed by the scholar Aristophanes of Byzantium ( 257 – 185/180 BC), who worked at the Musaeum in Alexandria during the third century BC. Aristophanes of Byzantium also was the first to divide poems into lines, rather than