Xi (letter) Xi (letter) Xi (uppercase Ξ, lowercase ξ; ) is the 14th letter of the Greek alphabet. It is pronounced in Modern Greek, and generally or in English. In the system of Greek numerals, it has a value of 60. Xi was derived from the Phoenician letter samekh . Xi is not to be confused with the letter chi, which gave its form to the Latin letter X. Both in classical Ancient Greek and in Modern Greek, the letter Ξ represents the sound . In some archaic local variants of the Greek alphabet, this letter was missing. Instead, especially in the
History of the Greek alphabet Phoenician values, but with different forms: as the Greek letter digamma for the consonant , and as the letter upsilon for the vowel . Upsilon was added at the end of the alphabet, perhaps to avoid upsetting the alphabetic order that was used in Greek numerals. Phoenician "hē" had been used as a "mater lectionis" for both and in addition to , but in Greek it was restricted to , following the acrophonic principle; its value was instead written with the letter "’āleph", while Greek was written with "ḥeth". All Phoenician letters had been acrophonic, and they remained so in