Xi ξ

Xi is the fourteenth Greek letter, from Phoenician samekh; its value is 60. Its one famous symbolic appearance is in Revelation 13:18, where the number of the beast, 666, is written with the three Greek numeral letters chi-xi-stigma; Xi supplies the middle digit, 60. The Greek letters doubled as numerals in the Milesian (alphabetic) system, in which each letter carried a fixed value; the practice of summing a word's letters into a number, called isopsephy, is the Greek counterpart of Hebrew gematria and appears in inscriptions, the Greek Magical Papyri and early Christian texts. Xi's Phoenician ancestry places it in a borrowing whose exact point of contact, somewhere among Al Mina, Cyprus, Crete and Euboea in the Aegean world, is still disputed, though the borrowing itself, conventionally dated to around the 8th century BCE, is agreed to be the ancestor of every European alphabet still in use, including the Latin one covered elsewhere in this atlas.

The Greeks' own contribution was to give vowels dedicated letters of their own, something the consonant-only Phoenician script never did. Its isopsephy value of 60 belongs to the Milesian alphabetic numeral system, which also needed three further, otherwise obsolete letters, digamma (also called stigma), koppa and sampi, to cover every value from 1 up to 900; none of the three survives among the 24 letters in ordinary use today.

Xi is one of the least visible letters of the Greek alphabet in everyday modern use, appearing mainly in specialised mathematical notation, where it commonly denotes a random variable in probability theory or an unspecified point in certain calculus theorems, alongside its use for a Bayer-designated star sharing the same ranked naming scheme as its neighbours. Its most widely reported recent appearance came in 2021, when the World Health Organization, working through its Greek-letter naming scheme for coronavirus variants, announced that it would skip xi, along with the preceding letter nu, when assigning the next variant name; WHO officials stated that xi was skipped specifically to avoid inadvertently naming a serious disease variant after a common surname, a concern that had nothing to do with the letter's ancient linguistic history, covered elsewhere in this atlas, or with its Semitic root samekh, meaning a prop or support.

The decision drew considerable public commentary at the time given its evident political sensitivity, making that single naming decision, rather than any classical or scientific use, xi's best-known moment in recent public discussion. In the Milesian numeral system, discussed elsewhere in this atlas, xi carries the value sixty, a use so specialised that most readers encounter the letter only through mathematics or through the 2021 naming controversy rather than through any everyday context.