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Tau (uppercase Τ, lowercase τ; Ταυ ˈtaf) 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 ˈtaʊ|, but in Modern Greek it is ˈtaf. 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).

Letters that arose from tau include Roman T and Cyrillic Te (Т, τ).

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.

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).


Scientific uses

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

  • The perceived motion-gap in General Tau Theory, a psychological principle of perception.
  • Torque, the rotational force in mechanics.
  • In the physical sciences, tau is sometimes used as time variable, to avoid confusing t as temperature.
  • Tuning and Analysis Utilities (TAU) performance evaluation tool, computer science [1]
  • Used as the symbol for tangent by Richard Feynman in order to avoid any confusion caused by "tan"
  • The tau lepton, an elementary particle in particle physics.
  • The lifetime of a spontaneous emission process.
  • The specific tax amount.
  • The time constant of any device, such as an RC circuit.
  • The dose interval in pharmacokinetics.
  • Proper time in relativity.
  • The symbol of continuity in electro-flux resonance.
  • The symbol for tortuosity in hydrogeology.
  • A coefficient of correlation — see Kendall's tau.
  • The golden ratio 1.618..., although φ (phi) is more common
  • Ramanujan's tau function in number theory.
  • The divisor function in number theory.
  • Tau in astronomy is a measure of opacity, or how much sunlight cannot penetrate the atmosphere.
  • Tau (protein) in biochemistry is a protein associated with microtubules and is implicated in neurodegenerative diseases such as Alzheimer's Disease and FTLD.
  • Shear stress in continuum mechanics.
  • The prefix of many stars, via the Bayer stellar designation system. (Tau Ceti is such a star.)
  • The expressed period of the freerunning rhythm of an animal (circadian rhythm terminology), i.e., the length of the daily cycle of an animal when kept in constant light or constant darkness
  • Kendall's tau rank correlation coefficient as a non-parametric correlation measure in statistics.
  • Torsion of a curve in differential geometry.

Other uses

  • In ancient times, 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/,
  • 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 neck.


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 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.


Majuscule form
Minuscule form

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