Xi (letter) dialects of most of the Greek mainland and Euboea, the sound [ks] was represented by Χ (which in classical Greek is chi, used for ). Because this variant of the Greek alphabet was used in Italy, the Latin alphabet borrowed Χ rather than Ξ 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). The uppercase letter Ξ is used as symbol for: The lowercase letter ξ is used as a symbol for: These characters are used only as mathematical symbols. Stylized Greek text should
History of the alphabet All of the names of the letters of the Phoenician alphabet started with consonants, and these consonants were what the letters represented, something called the acrophonic principle. However, several Phoenician consonants were absent in Greek, and thus several letter names came to be pronounced with initial vowels. Since the start of the name of a letter was expected to be the sound of the letter (the acrophonic principle), in Greek these letters came to be used for vowels. For example, the Greeks had no glottal stop or voiced pharyngeal sounds, so the Phoenician letters "’alep" and "`ayin" became Greek "alpha"