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Xi (uppercase Ξ, lowercase ξ) is the 14th letter of the Greek alphabet. It is pronounced ˈksi in Modern Greek, and generally ˈzaɪ or ˈsaɪ in English.[1] In the system of Greek numerals, it has a value of 60.

Xi is not to be confused with the letter chi, which gave its form to the Latin letter X.



A joined variant of Ξ.
A joined variant of Ξ.

Both in Modern Greek and in the modern system for writing Ancient Greek, xi represents /ks/. This usage comes from the regional variants of the Greek alphabet used to write the Attic and Ionic dialects.

In the Western or Euboean alphabet, the sounds represented by xi and chi were the opposite of the modern usage: xi represented /kʰ/, and chi (Χ) represented /ks/. Because this variant of the Greek alphabet was used in Italy, the Latin alphabet borrowed chi rather than xi as the Latin letter X.


While having no Latin derivative, the Xi was adopted into the early Cyrillic alphabet, as the letter ksi (Ѯ, ѯ).

Math and science

Upper case

The upper-case letter Ξ is used as symbol for:

Lower case

The lower-case letter ξ is used as a symbol for:


Majuscule form
Minuscule form

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