Editing Chi (letter)

{{Redirect|χ|the consonant represented by <code>χ</code> in IPA|Voiceless uvular fricative}}
{{Redirect-distinguish|Χ|X|×}}
{{Dist|Xi (letter)}}
{{Short description|Twenty-second letter of the Greek alphabet}}
{{Greek Alphabet|letter=chi}}
'''Chi''' ({{IPAc-en|audio=En-us-chi.ogg|k|aɪ}} {{respell|KY}}, also {{IPAc-en|x|i:}} {{respell|KHEE}};<ref name="Chambers">{{Cite book|title=The Chambers Dictionary|publisher=Chambers|year=2003|isbn=0-550-10105-5|edition=9th|chapter=chi}}</ref><ref>{{OED|chi}}</ref> uppercase '''Χ''', lowercase '''χ'''; {{langx|el|χῖ}}) is the twenty-second letter of the [[Greek alphabet]].

==Greek==
[[File:NAMA Alphabet grec.jpg|thumb|The Greek alphabet on a [[black figure]] vessel, with a cross-shaped chi]]

===Pronunciation===
====Ancient Greek====
Its value in [[Ancient Greek]] was an aspirated [[voiceless velar plosive|velar stop]] {{IPA|/kʰ/}} (in the [[Western Greek alphabet]]: /ks/).

====Koine Greek====
In [[Koine Greek]] and later dialects it became a [[Fricative consonant|fricative]] ({{IPA|[x]}}/{{IPA|[ç]}}) along with [[Theta (letter)|Θ]] and [[Phi (letter)|Φ]].

====Modern Greek====
In [[Modern Greek]], it has two distinct pronunciations: In front of [[close vowel|high]] or [[front vowel]]s ({{IPA|/e/}} or {{IPA|/i/}}) it is pronounced as a [[voiceless palatal fricative]] {{IPA|[ç]}}, as in [[Standard German phonology#Ich-Laut and ach-Laut|German ''ich'']] or like [[English phonology#Consonants|some pronunciations of "h"]] in English words like ''hew'' and ''human''. In front of [[open vowel|low]] or [[back vowel]]s ({{IPA|/a/}}, {{IPA|/o/}} or {{IPA|/u/}}) and [[consonants]], it is pronounced as a [[voiceless velar fricative]] ({{IPA|[x]}}), as in German ''ach'' or [[Spanish phonology#Consonants|Spanish ''j'']]. This distinction corresponds to the ich-Laut and ach-Laut of German.

===Transliteration===
Chi is [[Romanization of Greek|romanized]] as {{angle bracket|ch}} in most systematic transliteration conventions, but sometimes {{angle bracket|kh}} is used.<ref>{{Cite web |date=2024-12-20 |title=Greek language {{!}} Definition, Alphabet, Origin, & Facts {{!}} Britannica |url=https://www.britannica.com/topic/Greek-language |access-date=2025-02-09 |website=www.britannica.com |language=en |quote=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.}}</ref> In addition, in Modern Greek, it is often also romanized as {{angle bracket|h}} or {{angle bracket|x}} in informal practice.

===Greek numeral===
In the system of [[Greek numerals]], it has a value of 600.

===Xi===
In ancient times, some [[History of the Greek alphabet#Epichoric alphabets|local forms]] of the [[Greek alphabet]] used the chi instead of [[Xi (letter)|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 [[Kha (Cyrillic)|Х]], with the phonetic value /x/ or /h/.

==International Phonetic Alphabet==
In the [[International Phonetic Alphabet]], {{angbr IPA|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 (dialogue)|Timaeus]]'', it is explained that the two bands that form the [[Anima mundi (spirit)|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#.CE.A7|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. 

==Math and science==

* In [[statistics]], the term chi-squared or <math alt="χ²">\chi^2</math> has [[Chi-squared (disambiguation)|various uses]], including the [[chi-squared distribution]], the [[chi-squared test]], and [[chi-squared target models|chi-squared target model]]
* In [[algebraic topology]], Chi is used to represent the [[Euler characteristic]] of a surface.<ref>{{Cite web |last=Weisstein |first=Eric W. |title=Euler Characteristic |url=https://mathworld.wolfram.com/EulerCharacteristic.html |access-date=2025-02-09 |website=mathworld.wolfram.com |language=en}}</ref>
* The [[chromatic number]] of a graph in [[graph theory]]<ref>{{Cite web |last=Weisstein |first=Eric W. |title=Chromatic Number |url=https://mathworld.wolfram.com/ChromaticNumber.html |access-date=2025-02-09 |website=mathworld.wolfram.com |language=en |quote=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),...}}</ref>
* In [[neuroanatomy]], [[Chiasm (anatomy)|crossings]] of [[Peripheral nervous system|peripheral nerves]] (such as the [[optic chiasm]]) are named for the letter Chi because of its Χ-shape.<ref>{{cite book |title=The Human Brain |last=Asimov |first=Isaac |author-link= Isaac Asimov |year=1963 |publisher=Houghton Mifflin |location=Boston}}</ref>
* In [[chemistry]], the [[mole fraction]]<ref>{{cite book|last=Zumdahl|first=Steven S.|title=Chemistry|url=https://archive.org/details/chemistrythediti00zumd|url-access=limited|year=2008|publisher=Cengage Learning|isbn=978-0547125329|edition=8th|page=[https://archive.org/details/chemistrythediti00zumd/page/n229 201]}}</ref><ref>{{cite book|first1=James N.|last1=Spencer|first2=George M.|last2=Bodner|first3=Lyman H.|last3=Rickard|title=Chemistry: structure and dynamics|url=https://archive.org/details/chemistrystructu00spen_759|url-access=limited|year=2010|publisher=Wiley|location=Hoboken, NJ|isbn=9780470587119|edition=5th|page=[https://archive.org/details/chemistrystructu00spen_759/page/n378 357]}}</ref> and [[electronegativity]]<ref>{{GoldBookRef|file=E01990|title=Electronegativity}}</ref> may be denoted by the lowercase <math alt="χ">\chi</math>.
* In [[physics]], <math alt="χ">\chi</math> denotes [[electric susceptibility|electric]] or [[magnetic susceptibility]].<ref>{{Cite journal |last=Mugiraneza |first=Sam |last2=Hallas |first2=Alannah M. |date=2022-04-19 |title=Tutorial: a beginner’s guide to interpreting magnetic susceptibility data with the Curie-Weiss law |url=https://www.nature.com/articles/s42005-022-00853-y |journal=Communications Physics |language=en |volume=5 |issue=1 |pages=1–12 |doi=10.1038/s42005-022-00853-y |issn=2399-3650 |quote=However, for newly synthesized materials, there is one indispensable characterization technique that is as old as the field of magnetism itself: magnetic susceptibility, χ,...|arxiv=2205.07107 }}</ref><ref>{{Cite book |last=Jiles |first=David |title=Introduction to magnetism and magnetic materials |date=2016 |publisher=CRC Press, Taylor & Francis Group |isbn=978-1-138-44149-1 |edition=Third |location=Boca Raton London New York |pages=42 |quote=We can now make a general statement for the permeability ''μ'' and susceptibility ''{{mvar|χ}}''}}</ref>
* 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==

* {{unichar|03A7|html=}}<ref>Unicode Code Charts: [https://www.unicode.org/charts/PDF/U0370.pdf Greek and Coptic (Range: 0370-03FF)]</ref>
* {{unichar|03C7|html=}}
* {{unichar|1D61|html=}}
* {{unichar|1D6A|html=}}
* {{unichar|2627|html=}}
* {{unichar|2CAC|html=}}
* {{unichar|2CAD|html=}}
* {{unichar|2CE9|html=}}
* {{unichar|A7B3|html=}}
* {{unichar|AB53|html=}}
* {{unichar|AB54|html=}}
* {{unichar|AB55|html=}}
* {{unichar|1D6BE|html=}}
* {{unichar|1D6D8|html=}}
* {{unichar|1D6F8|html=}}
* {{unichar|1D712|html=}}
* {{unichar|1D732|html=}}
* {{unichar|1D74C|html=}}
* {{unichar|1D76C|html=}}
* {{unichar|1D786|html=}}
* {{unichar|1D7A6|html=}}
* {{unichar|1D7C0|html=}}

==See also==
{{Wiktionary|Χ|χ}}
*[[Chi (disambiguation)]]
*[[Kha (Cyrillic)|Х, х - Kha (Cyrillic)]]

==References==
<references/>

[[Category:Greek letters]]
[[Category:Phonetic transcription symbols]]