Simonides of Ceos
Plutarch and other ancient Greek writers credited the legendary Palamedes of Nauplion on Euboea with the invention of the supplementary letters not found in the original Phoenician alphabet. The distinction between "Eta" and "Epsilon" and between "Omega" and "Omicron", adopted in the Ionian standard, was traditionally attributed to Simonides of Ceos (556-469).