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 ranked letter. Several Greek letters are used as phonetic symbols in the International Phonetic Alphabet (IPA). Several of them denote fricative consonants; the rest stand for variants of vowel sounds. The glyph shapes used for these letters in specialized phonetic fonts is sometimes slightly different from the conventional shapes in Greek typography proper, with glyphs typically being more upright and using serifs, to make them conform more with the typographical character of other, Latin-based letters in the phonetic alphabet. Nevertheless, in the Unicode encoding standard, the following three phonetic symbols are considered the same characters as the corresponding Greek letters