Tau (uppercase Τ, lowercase τ; el, ταυ ) is the 19th letter of the . In the system of , it has a value of 300. The name in English is pronounced or , but in 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 ). Tau was derived from the (𐤕). Letters that arose from tau include Roman and Cyrillic (Т, т). The letter occupies the slots U+03C4 (lowercase) and U+03A4 (uppercase). In , they can be produced with (τ and Τ), (τ and Τ), or (τ and Τ).

Modern usage

The lower-case letter τ is used as a symbol for: * amount


* The expressed period of the rhythm of an animal, i.e., the length of the daily cycle of an animal when kept in constant light or constant darkness * The dose interval in * The core variable in * in biochemistry, a protein associated with s and implicated in neurodegenerative diseases such as , some forms of , and


* in number theory, also denoted ''d'' or σ0The date given on the source is after that of the archive. This is because the original publishing date is unknown, so the latest update date is stated instead. * (1.618...), although φ () is more common * in statistics * in stochastic processes.The archived version of this source may take a few minutes to render the math codes properly. * , circle constant equal to 2 (6.28318...) * s, several * in differential geometry * in Euclidean geometry (although the Latin letter is used more often) * The


* in relativity * in continuum mechanics * The lifetime of a process * , an elementary particle in particle physics * Tau in is a measure of , or how much sunlight cannot penetrate the atmosphere * In the physical sciences, tau is sometimes used as variable, to avoid confusing t as temperature * (also ) of any system, such as an * , the rotational force in mechanics * The symbol for in hydrogeology


* In ancient times, tau was used as a symbol for or , whereas the eighth letter of the Greek alphabet, , was considered the symbol of . * In Biblical times, the was put on men to distinguish those who lamented sin, although newer versions of the Bible have replaced the ancient term ''taw'' with ''mark'' (Ezekiel 9:4) or ''signature'' (Job 31:35). Its original sound value is a , IPA /t/ * The symbolism of the was connected not only to the letter but also to tau, the equivalent of the last letter in the and Old Hebrew alphabets, and which was originally cruciform in shape; see . * An essay written around 160 AD, attributed to , a mock legal prosecution called ', contains a reference to the cross attribution. petitions the court to sentence Tau to death by crucifixion, saying: *: ''Men weep, and bewail their lot, and curse Cadmus with many curses for introducing ''Tau'' into the family of letters; they say it was his body that tyrants took for a model, his shape that they imitated, when they set up structures on which men are crucified. ''Stauros'' (cross) the vile engine is called, and it derives its vile name from him. Now, with all these crimes upon him, does he not deserve death, nay, many deaths? For my part I know none bad enough but that supplied by his own shape — that shape which he gave to the named ''stauros'' after him by men'' * is usually considered as the symbol of orders due to love for it, symbol of the redemption and of the . Almost all Franciscan churches have painted a tau with two crossing arms, both with , the one of Jesus and the other of Francis; members of the usually wear a wooden τ in a string with three knots around the neck * The title and symbol of "Tau" is used by bishops as it has some symbolism in many of the modern branches of Gnosticism.

Character encodings

For the Greek and Coptic letter tau: For the mathematical letter tau: These characters are used only as mathematical symbols. Stylized Greek text should be encoded using the normal Greek letters, with markup and formatting to indicate text style.

See also




{{Authority control Cross symbols