Digamma or Wau (uppercase/lowercase Ϝ ϝ ς) was an old letter of the Greek alphabet. It was used before the alphabet converted its classical standard form. It looked like a Latin "F", but it was pronounced like "w". In the 5th century BC, people stopped using it because they could no longer pronounce the sound "w" in Greek. However, they kept it as a sign for the number "6" in the system of Greek numerals.