# Chi (letter)

Chi (uppercase Χ, lowercase χ; Greek: χῖ) is the 22nd letter of the Greek alphabet, pronounced /k/ or /k/ in English.

## 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], as in German ich or like the h in some pronunciations of the English words hew and human. In front of low or back vowels (/a/, /o/ or /u/) and consonants, it is pronounced as a voiceless velar fricative ([x]), as in German ach.

### Transliteration

Chi is romanized as ch in most systematic transliteration conventions, but sometimes kh is used.[1] 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, the minuscule chi is the symbol for the 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 typespace with the Greek letter Rho, it is called the labarum and used to represent the person of Jesus Christ.

## Character encodings

### Greek chi

CharacterΧχ
Unicode nameGREEK CAPITAL LETTER CHIGREEK SMALL LETTER CHIMODIFIER LETTER SMALL CHIGREEK SUBSCRIPT SMALL LETTER CHICHI RHO
Encodingsdecimalhexdecimalhexdecimalhexdecimalhexdecimalhex
Unicode935U+03A7967U+03C77521U+1D617530U+1D6A9767U+2627
UTF-8206 167CE A7207 135CF 87225 181 161E1 B5 A1225 181 170E1 B5 AA226 152 167E2 98 A7
Numeric character reference&#935;&#x3A7;&#967;&#x3C7;&#7521;&#x1D61;&#7530;&#x1D6A;&#9767;&#x2627;
Named character reference&Chi;&chi;
DOS Greek-2210D2243F3
Windows 1253214D6246F6
TeX\chi

[2]

### Coptic khi

Character
Unicode nameCOPTIC CAPITAL LETTER KHICOPTIC SMALL LETTER KHICOPTIC SYMBOL KHI RHO
Encodingsdecimalhexdecimalhexdecimalhex
UTF-8226 178 172E2 B2 AC226 178 173E2 B2 AD226 179 169E2 B3 A9

### Latin chi

Character
Unicode nameLATIN CAPITAL LETTER CHILATIN SMALL LETTER CHILATIN SMALL LETTER CHI WITH LOW RIGHT RINGLATIN SMALL LETTER CHI WITH LOW LEFT SERIF
Encodingsdecimalhexdecimalhexdecimalhexdecimalhex
Unicode42931U+A7B343859U+AB5343860U+AB5443861U+AB55
UTF-8234 158 179EA 9E B3234 173 147EA AD 93234 173 148EA AD 94234 173 149EA AD 95
Numeric character reference&#42931;&#xA7B3;&#43859;&#xAB53;&#43860;&#xAB54;&#43861;&#xAB55;

### Mathematical chi

Character𝚾𝛘𝛸𝜒𝜲𝝌
Unicode nameMATHEMATICAL BOLD
CAPITAL CHI
MATHEMATICAL BOLD
SMALL CHI
MATHEMATICAL ITALIC
CAPITAL CHI
MATHEMATICAL ITALIC
SMALL CHI
MATHEMATICAL BOLD ITALIC
CAPITAL CHI
MATHEMATICAL BOLD ITALIC
SMALL CHI
Encodingsdecimalhexdecimalhexdecimalhexdecimalhexdecimalhexdecimalhex
Unicode120510U+1D6BE120536U+1D6D8120568U+1D6F8120594U+1D712120626U+1D732120652U+1D74C
UTF-8240 157 154 190F0 9D 9A BE240 157 155 152F0 9D 9B 98240 157 155 184F0 9D 9B B8240 157 156 146F0 9D 9C 92240 157 156 178F0 9D 9C B2240 157 157 140F0 9D 9D 8C
UTF-1655349 57022D835 DEBE55349 57048D835 DED855349 57080D835 DEF855349 57106D835 DF1255349 57138D835 DF3255349 57164D835 DF4C
Numeric character reference&#120510;&#x1D6BE;&#120536;&#x1D6D8;&#120568;&#x1D6F8;&#120594;&#x1D712;&#120626;&#x1D732;&#120652;&#x1D74C;
Character𝝬𝞆𝞦𝟀
Unicode nameMATHEMATICAL SANS-SERIF
BOLD CAPITAL CHI
MATHEMATICAL SANS-SERIF
BOLD SMALL CHI
MATHEMATICAL SANS-SERIF
BOLD ITALIC CAPITAL CHI
MATHEMATICAL SANS-SERIF
BOLD ITALIC SMALL CHI
Encodingsdecimalhexdecimalhexdecimalhexdecimalhex
Unicode120684U+1D76C120710U+1D786120742U+1D7A6120768U+1D7C0
UTF-8240 157 157 172F0 9D 9D AC240 157 158 134F0 9D 9E 86240 157 158 166F0 9D 9E A6240 157 159 128F0 9D 9F 80
UTF-1655349 57196D835 DF6C55349 57222D835 DF8655349 57254D835 DFA655349 57280D835 DFC0
Numeric character reference&#120684;&#x1D76C;&#120710;&#x1D786;&#120742;&#x1D7A6;&#120768;&#x1D7C0;

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 ${\displaystyle \chi ^{2}}$ has 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 neurology, the optic chiasm is named for the letter Chi because of its Χ-shape.[3]

In chemistry, the mole fraction[4][5] and electronegativity[6] may be denoted by the lowercase ${\displaystyle \chi }$.

In physics, ${\displaystyle \chi }$ denotes 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 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.