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
Lydian alphabet instead. Lydian uniquely features a quotation mark in the shape of a right triangle. The Lydian alphabet is closely related to the other alphabets of Asia Minor as well as to the Greek alphabet. It contains letters for 26 sounds. Some are represented by more than one symbol, which is considered one "letter." Unlike the Carian alphabet, which had an "f" derived from Φ, the Lydian "f" has the peculiar "8" shape also found in the Etruscan alphabet. In addition two digraphs, "aa" and "ii", appear to be allophones of [a] and [i] under speculative circumstances, such as lengthening from