Chi (letter)

{{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|,_|"|x|i:}};<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'''; {{lang-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>[https://www.britannica.com/EBchecked/topic/3390/Achaea Encyclopædia Britannica]</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|ꭓ}} 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. 

==Character encodings==
===Greek chi===

{{charmap
|03A7|name1=Greek Capital Letter Chi
|03C7|name2=Greek Small Letter Chi
|1D61|name3=Modifier Letter Small Chi
|1D6A|name4=Greek Subscript Small Letter Chi
|2627|name5=Chi Rho
|map2=DOS Greek|map2char1=94|map2char2=AD
|map3=DOS Greek-2|map3char1=D2|map3char2=F3
|map4=Windows 1253|map4char1=D6|map4char2=F6
|namedref1=[[TeX]]|ref1char2=\chi
}}
<ref>Unicode Code Charts: [https://www.unicode.org/charts/PDF/U0370.pdf Greek and Coptic (Range: 0370-03FF)]</ref>

===Coptic khi===
{{charmap
|2CAC|name1=Coptic Capital Letter Khi
|2CAD|name2=Coptic Small Letter Khi
|2CE9|name3=Coptic Symbol Khi Rho
}}

===Latin chi===
{{charmap
|A7B3|name1=Latin Capital Letter Chi
|AB53|name2=Latin Small Letter Chi
|AB54|name3=Latin Small Letter Chi With Low Right Ring
|AB55|name4=Latin Small Letter Chi With Low Left Serif
}}

===Mathematical chi===
{{charmap
|1D6BE|name1=Mathematical Bold<br />Capital Chi
|1D6D8|name2=Mathematical Bold<br />Small Chi
|1D6F8|name3=Mathematical Italic<br />Capital Chi
|1D712|name4=Mathematical Italic<br />Small Chi
|1D732|name5=Mathematical Bold Italic<br />Capital Chi
|1D74C|name6=Mathematical Bold Italic<br />Small Chi
}}

{{charmap
|1D76C|name1=Mathematical Sans-Serif<br />Bold Capital Chi
|1D786|name2=Mathematical Sans-Serif<br />Bold Small Chi
|1D7A6|name3=Mathematical Sans-Serif<br />Bold Italic Capital Chi
|1D7C0|name4=Mathematical Sans-Serif<br />Bold Italic Small Chi
}}

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.

==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]].

In [[algebraic topology]], Chi is used to represent the [[Euler characteristic]] of a surface.

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|last=Rickard|first=James N. Spencer, George M. Bodner, Lyman H.|title=Chemistry : structure and dynamics.|url=https://archive.org/details/chemistrystructu00spen_759|url-access=limited|year=2010|publisher=Wiley|location=Hoboken, N.J.|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]].

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 [[graph theory]], a lowercase chi is used to represent a graph's [[chromatic number]].

In [[analytic number theory]], chi is used for the [[Dirichlet character]].

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

==References==
<references/>

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