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
History of the Greek alphabet (letter). In the 8th to 6th centuries, local or "epichoric" variants of the alphabet developed. They are classified into three main groups, following Adolf Kirchhoff (1887): green (Cretan), red (Euboean or Western) and blue (Ionic, Attic and Corinthian). The main distinction is in the supplemental signs added to the Phoenician core inventory. With the exception of the early Fayum alphabet, which does not fit into the tripartite scheme, all abecedaries add Υ to the Phoenician inventory. The green alphabets have only this; the red add Φ for , Χ for , and Ψ for ; and the blue add Φ