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Chi (uppercase Χ, lowercase χ; χῖ) is the 22nd letter of the Greek alphabet, pronounced as ˈkaɪ in English.




Ancient Greek


Koine Greek

In Koine Greek and later dialects it became a fricative 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. Chi is Romanized as <ch> in most systematic transliteration conventions, but also sometimes as <h> or <x> in informal practice.

Greek numeral



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 letter X being used for the same sound in Latin, and the modern languages which use the Latin alphabet.


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

International Phonetic Alphabet

In the International Phonetic Alphabet, chi is the symbol for the voiceless uvular fricative.


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




Math and science

Chi is also frequently used in statistics.

In Algebraic Topology, Chi is used to represent the Euler Characteristic of a surface.

The optic chiasm, an x-shaped connection of the optic nerves leading to the eye, got its name from the letter chi because of its shape;[1] likewise, the shape of the letter chi is the origin of the rhetorical device chiasmus.

In EN 1993, an European Standard for the design of steel structures, the chi character is used as a symbol for the reduction factor for the relevant buckling load.


  • 1. Asimov, Isaac (1963). The Human Brain. Boston: Houghton Mifflin.

See also

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