Digamma (uppercase Ϝ, lowercase ϝ) is an archaic letter of the Greek alphabet, used primarily as a Greek numeral.
The letter had the phonetic value of a voiced labial-velar approximant /w/. It was originally called Ϝαῦ wau. It was later called διγαμμα (digamma — double gamma) because of its shape. It is attested in archaic and dialectal ancient Greek inscriptions, and is occasionally used as a symbol in later Greek mathematical texts.
Digamma, like Upsilon, derives from the Phoenician letter Waw, and in its turn gave rise to the Roman letter F.

The sound /w/ in Greek