Chi (letter)

Chi (/kaɪ/ ^ⓘ KY, also /xiː/ KHEE;^[1]^[2] uppercase Χ, lowercase χ; Greek: χῖ) is the twenty-second letter of the Greek alphabet.

Greek

Pronunciation

Ancient Greek

Its value in Ancient Greek was an aspirated velar stop /kʰ/ (in the Western Greek alphabet: /ks/).

Koine Greek

In Koine Greek and later dialects it became a fricative ([x]/[ç]) along with Θ and Φ.

Modern Greek

In Modern Greek, it has two distinct pronunciations: In front of high or front vowels (/e/ or /i/) it is pronounced as a voiceless palatal fricative [ç], which sounds similar to "h" in English words like hew and human and is equivalent to the German ich-Laut as in dich. In front of low or back vowels (/a/, /o/ or /u/) and consonants, it is pronounced as a voiceless velar fricative ([x]), like the German ach-Laut as in Bach or the Spanish j.

Transliteration

Chi is romanized as ⟨ch⟩ in most systematic transliteration conventions, but sometimes ⟨kh⟩ is used.^[3] In addition, in Modern Greek, it is often also romanized as ⟨h⟩ or ⟨x⟩ in informal practice.

Greek numeral

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

Xi

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

Cyrillic

Chi was also included in the Cyrillic script as the letter Х, with the phonetic value /x/ or /h/.

International Phonetic Alphabet

In the International Phonetic Alphabet, ⟨χ⟩ represents a voiceless uvular fricative.

Chiasmus

Chi is the basis for the name literary chiastic structure and the name of chiasmus.

Symbolism

In Plato's Timaeus, it is explained that the two bands that form the soul of the world cross each other like the letter Χ. Plato's analogy, along with several other examples of chi as a symbol occur in Thomas Browne's discourse The Garden of Cyrus (1658).

Chi or X is often used to abbreviate the name Christ, as in the holiday Christmas (Xmas). When fused within a single typeface with the Greek letter rho, it is called the Chi Rho and used to represent the person of Jesus Christ.

Mathematics and science

In statistics, the term chi-squared or $\chi ^{2}$ $\chi ^{2}$ has various uses, including the chi-squared distribution, the chi-squared test, and chi-squared target model
In algebraic topology, Chi is used to represent the Euler characteristic of a surface.^[4]
The chromatic number of a graph in graph theory^[5]
In neuroanatomy, crossings of peripheral nerves (such as the optic chiasm) are named for the letter Chi because of its Χ-shape.^[6]
In chemistry, the mole fraction^[7]^[8] and electronegativity^[9] may be denoted by the lowercase $\chi$ $\chi$ .
In physics, $\chi$ $\chi$ denotes electric or magnetic susceptibility.^[10]^[11]
In rhetoric, both chiastic structure (a literary device) and the figure of speech Chiasmus derive from their names from the shape of the letter Chi.
In mechanical engineering, chi is used as a symbol for the reduction factor of relevant buckling loads in the EN 1993, a European Standard for the design of steel structures.
In analytic number theory, chi is used for the Dirichlet character.

Unicode

U+03A7 Χ GREEK CAPITAL LETTER CHI (Χ)^[12]
U+03C7 χ GREEK SMALL LETTER CHI (χ)
U+1D61 ᵡ MODIFIER LETTER SMALL CHI
U+1D6A ᵪ GREEK SUBSCRIPT SMALL LETTER CHI
U+2627 ☧ CHI RHO
U+2CAC Ⲭ COPTIC CAPITAL LETTER KHI
U+2CAD ⲭ COPTIC SMALL LETTER KHI
U+2CE9 ⳩ COPTIC SYMBOL KHI RO
U+A7B3 Ꭓ LATIN CAPITAL LETTER CHI
U+AB53 ꭓ LATIN SMALL LETTER CHI
U+AB54 ꭔ LATIN SMALL LETTER CHI WITH LOW RIGHT RING
U+AB55 ꭕ LATIN SMALL LETTER CHI WITH LOW LEFT SERIF
U+1D6BE 𝚾 MATHEMATICAL BOLD CAPITAL CHI
U+1D6D8 𝛘 MATHEMATICAL BOLD SMALL CHI
U+1D6F8 𝛸 MATHEMATICAL ITALIC CAPITAL CHI
U+1D712 𝜒 MATHEMATICAL ITALIC SMALL CHI
U+1D732 𝜲 MATHEMATICAL BOLD ITALIC CAPITAL CHI
U+1D74C 𝝌 MATHEMATICAL BOLD ITALIC SMALL CHI
U+1D76C 𝝬 MATHEMATICAL SANS-SERIF BOLD CAPITAL CHI
U+1D786 𝞆 MATHEMATICAL SANS-SERIF BOLD SMALL CHI
U+1D7A6 𝞦 MATHEMATICAL SANS-SERIF BOLD ITALIC CAPITAL CHI
U+1D7C0 𝟀 MATHEMATICAL SANS-SERIF BOLD ITALIC SMALL CHI

References

"chi". The Chambers Dictionary (9th ed.). Chambers. 2003. ISBN 0-550-10105-5.
"chi". Oxford English Dictionary (Online ed.). Oxford University Press. (Subscription or participating institution membership required.)
"Greek language | Definition, Alphabet, Origin, & Facts | Britannica". www.britannica.com. 2024-12-20. Some differences in transliteration result from changes in pronunciation of the Greek language; others reflect convention, as for example the χ (chi or khi), which was transliterated by the Romans as ch (because they lacked the letter k in their usual alphabet). In Modern Greek, however, the standard transliteration for χ is kh.
Weisstein, Eric W. "Euler Characteristic". mathworld.wolfram.com.
Weisstein, Eric W. "Chromatic Number". mathworld.wolfram.com. The chromatic number of a graph G is most commonly denoted χ (G) (e.g., Skiena 1990, West 2000, Godsil and Royle 2001, Pemmaraju and Skiena 2003),...
Asimov, Isaac (1963). The Human Brain. Boston: Houghton Mifflin.
Zumdahl, Steven S. (2008). Chemistry (8th ed.). Cengage Learning. p. 201. ISBN 978-0-547-12532-9.
Spencer, James N.; Bodner, George M.; Rickard, Lyman H. (2010). Chemistry: structure and dynamics (5th ed.). Hoboken, New Jersey: Wiley. p. 357. ISBN 978-0-470-58711-9.
IUPAC, Compendium of Chemical Terminology, 5th ed. (the "Gold Book") (2025). Online version: (2006–) "Electronegativity". doi:10.1351/goldbook.E01990
Mugiraneza, Sam; Hallas, Alannah M. (2022-04-19). "Tutorial: a beginner's guide to interpreting magnetic susceptibility data with the Curie-Weiss law". Communications Physics. 5 (1): 1–12. arXiv:2205.07107. doi:10.1038/s42005-022-00853-y. ISSN 2399-3650. However, for newly synthesized materials, there is one indispensable characterization technique that is as old as the field of magnetism itself: magnetic susceptibility, χ,...
Jiles, David (2016). Introduction to magnetism and magnetic materials (Third ed.). Boca Raton London New York: CRC Press, Taylor & Francis Group. p. 42. ISBN 978-1-138-44149-1. We can now make a general statement for the permeability μ and susceptibility χ
Unicode Code Charts: Greek and Coptic (Range: 0370-03FF)