Chi (letter)

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Greek alphabet
Α α Alpha	Ν ν Nu
Β β Beta	Ξ ξ Xi
Γ γ Gamma	Ο ο Omicron
Δ δ Delta	Π π ϖ Pi
Ε ε Epsilon	Ρ ρ Rho
Ζ ζ Zeta	Σ σ ς Sigma
Η η Eta	Τ τ Tau
Θ θ Theta	Υ υ Upsilon
Ι ι Iota	Φ φ Phi
Κ κ Kappa	Χ χ Chi
Λ λ Lambda	Ψ ψ Psi
Μ μ Mu	Ω ω Omega
Obsolete letters
Ϝ ϝ Digamma	Ϻ ϻ San
Ϙ ϙ Qoppa	Ϡ ϡ Sampi
Ϛ ϛ Stigma	Ϸ ϸ Sho
diacritics

For other uses, see Chi.

Chi (uppercase Χ, lowercase χ) is the 22nd letter of the Greek alphabet. Its value in Ancient Greek was an aspirated velar stop (/kʰ/), but in Koine Greek and later dialects it fricativized along with Θ and Φ. In Modern pronunciation, it is a voiceless dorsal fricative. In front of high or front vowels (e, i, oi, ai, y) it is prounounced like the German ich-laut (/ç/, voiceless palatal fricative) or some variants of 'h' in the English 'hew' or 'human'. In front of back or low vowels (a, o, ou) and consonants, it is pronounced like the German ach-laut (/χ/, voiceless uvular fricative) or in Scottish 'loch'.

In the system of Greek numerals it has a value of 600.

In ancient times, some dialects of Greek used the chi instead of xi to represent the /ks/ sound. This was borrowed into the early Latin language, which led to the letter X being used for the same sound in Latin, and the modern languages which use the Latin alphabet.

Chi also was included into Cyrillic alphabet (as letter Х), with the phonetic value /χ/ or /h/.

The upper-case letter Χ is used as the symbol for:

• Part of or a subsitution for the Labarum monogram.
• The name of Jesus Christ in Christianity, as in X-mas etc.
• The last letter of the name of the TeX typesetting system.

The lower-case letter χ is used as the symbol for:

• In probability theory, chi distribution a continuous probability distribution.
• In hypothesis testing the chi-square distribution or Chi-square test, a commonly used test statistic.
• The electric susceptibility (sometimes with a subscript e) and magnetic susceptibility in physics.
• The chromatic number and the chromatic index (when shown with a prime mark or a subscript 1) of a graph in graph theory.
• The Euler characteristic in algebraic topology.
• The voiceless uvular fricative (IPA) in phonology.
• An unsigned gathering in formal bibliographic collation.
• The mole fraction of a solution in chemistry.
• The characteristic function of a subset in mathematics
• The characteristic polynomial of a square matrix.

Chi is the basis for the name Chiastic structure and the name of Chiasmus.

In Plato's Timaeus, it is explained that the two bands which form the soul of the world cross each other like the letter Χ.