Tau (uppercase Τ, lowercase τ; Greek: ταυ ) 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/, but in modern Greek it is . This is because the pronunciation of the combination of Greek letters αυ has changed from ancient to modern times from one of to either or, depending on what follows (see Greek orthography).

Tau was derived from the Phoenician letter taw . Letters that arose from tau include Roman T and Cyrillic Te (Т, т).

The letter occupies the Unicode slots U+03C4 (lowercase) and U+03A4 (uppercase). In HTML, they can be produced with named entities (τ and Τ), decimal references (τ and Τ), or hexadecimal references (τ and Τ).

Read more about Tau:  Modern Usage, Symbolism