Tau (uppercase Τ, lowercase τ; Greek: ταυ [taf]) is the 19th letter of the Greek alphabet. In the system of Greek numerals it has a value of 300.The name in English is pronounced /taʊ/ or /tɔː/, but in modern Greek it is [taf]. This is because the pronunciation of the combination of Greek letters αυ has changed from ancient to modern times from one of [au] to either [av] or [af], depending on what follows (see Greek orthography).Tau was derived from the Phoenician letter taw 20px.