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 Phoenician values, but with different forms: as the Greek letter digamma for the consonant , and as the letter upsilon for the vowel . Upsilon was added at the end of the alphabet, perhaps to avoid upsetting the alphabetic order that was used in Greek numerals. Phoenician "hē" had been used as a "mater lectionis" for both and in addition to , but in Greek it was restricted to , following the acrophonic principle; its value was instead written with the letter "’āleph", while Greek was written with "ḥeth". All Phoenician letters had been acrophonic, and they remained so in