Digamma, waw, or wau (uppercase: , lowercase: , numeral: ) is an archaic letter of the Greek alphabet. It originally stood for the sound but it has principally remained in use as a Greek numeral for 6. Whereas it was originally called waw or wau, its most common appellation in classical Greek is digamma; as a numeral, it was called episēmon during the Byzantine era and is now known as stigma after the value of the Byzantine ligature combining σ-τ as ϛ.