|Look up Τ or τ in Wiktionary, the free dictionary.|
|Archaic local variants|
|In other languages|
The name in English is pronounced //, 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).
The letter occupies the Unicode slots U+03C4 (lowercase) and U+03A4 (uppercase). In HTML, they can be produced with named entities (
Τ), decimal references (
Τ), or hexadecimal references (
Modern usage 
The lower-case letter τ is used as a symbol for:
- The golden ratio (1.618...), although φ (phi) is more common.
- Tau-function in number theory.
- The divisor function in number theory, also denoted d or σ0.
- Torsion of a curve in differential geometry.
- Stopping time in stochastic processes
- Kendall tau rank correlation coefficient in statistics.
- Tau (2π) (6.283...), a proposed name for the ratio constant of a circle's circumference to radius.
- In the physical sciences, tau is sometimes used as time variable, to avoid confusing t as temperature.
- The tau lepton, an elementary particle in particle physics.
- Tau in astronomy is a measure of optical depth, or how much sunlight cannot penetrate the atmosphere.
- Proper time in relativity.
- The symbol for tortuosity in hydrogeology.
- Torque, the rotational force in mechanics.
- The lifetime of a spontaneous emission process.
- The prefix of many stars, via the Bayer stellar designation system. (Tau Ceti is such a star.)
- Shear stress in continuum mechanics.
- The time constant of any device, such as an RC circuit.
- The dose interval in pharmacokinetics.
- Tau (protein) in biochemistry is a protein associated with microtubules and is implicated in neurodegenerative diseases such as Alzheimer's disease, some forms of Frontotemporal lobar degeneration, and Chronic traumatic encephalopathy.
- The expressed period of the freerunning rhythm of an animal, i.e., the length of the daily cycle of an animal when kept in constant light or constant darkness.
- The core variable in General Tau Theory.
- In ancient times, Tau was used as a symbol for life and/or resurrection, whereas the eighth letter of the Greek alphabet, theta, was considered the symbol of death.
- In Biblical times, the Taw 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 voiceless alveolar plosive, IPA /t/.
- The symbolism of the cross was connected not only to the letter chi, but also to tau, the equivalent of the last letter in the Phoenician and Old Hebrew alphabets, and which was originally cruciform in shape; see Cross of Tau.
- An essay written around 160 AD, attributed to Lucian, a mock legal prosecution called The Consonants at Law — Sigma v. Tau in the Court of Seven Vowels contains a reference to the cross attribution. Sigma 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 gibbet named stauros after him by men.
- Tau is usually considered as the symbol of Franciscan orders due to St. Francis' love for it, symbol of the redemption and of the Cross. Almost all Franciscan churches have painted a tau with two crossing arms, both with stigmata, the one of Jesus and the other of Francis; usually members of the Secular Franciscan Order wear a wooden τ in a string with three knots around the ankle.
Character Encodings 
- Greek / Coptic Tau
|Unicode name||GREEK CAPITAL LETTER TAU||GREEK SMALL LETTER TAU||COPTIC CAPITAL LETTER TAU||COPTIC SMALL LETTER TAU|
|UTF-8||206 164||CE A4||207 132||CF 84||226 178 166||E2 B2 A6||226 178 167||E2 B2 A7|
|Numeric character reference||Τ||Τ||τ||τ||Ⲧ||Ⲧ||ⲧ||ⲧ|
|Named character reference||Τ||τ|
- Mathematical Tau
|Unicode name||MATHEMATICAL BOLD
|MATHEMATICAL BOLD ITALIC
|MATHEMATICAL BOLD ITALIC
|UTF-8||240 157 154 187||F0 9D 9A BB||240 157 155 149||F0 9D 9B 95||240 157 155 181||F0 9D 9B B5||240 157 156 143||F0 9D 9C 8F||240 157 156 175||F0 9D 9C AF||240 157 157 137||F0 9D 9D 89|
|UTF-16||55349 57019||D835 DEBB||55349 57045||D835 DED5||55349 57077||D835 DEF5||55349 57103||D835 DF0F||55349 57135||D835 DF2F||55349 57161||D835 DF49|
|Numeric character reference||𝚻||𝚻||𝛕||𝛕||𝛵||𝛵||𝜏||𝜏||𝜯||𝜯||𝝉||𝝉|
|Unicode name||MATHEMATICAL SANS-SERIF
BOLD CAPITAL TAU
BOLD SMALL TAU
BOLD ITALIC CAPITAL TAU
BOLD ITALIC SMALL TAU
|UTF-8||240 157 157 169||F0 9D 9D A9||240 157 158 131||F0 9D 9E 83||240 157 158 163||F0 9D 9E A3||240 157 158 189||F0 9D 9E BD|
|UTF-16||55349 57193||D835 DF69||55349 57219||D835 DF83||55349 57251||D835 DFA3||55349 57277||D835 DFBD|
|Numeric character reference||𝝩||𝝩||𝞃||𝞃||𝞣||𝞣||𝞽||𝞽|
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.