In Ancient Greek, 'Χ' and 'Ψ' were among several variants of the same letter, used originally for /kʰ/ and later, in western areas such as Arcadia, as a simplification of the digraph 'ΧΣ' for /ks/. In the end, more conservative eastern forms became the standard of Classical Greek, and thus 'Χ' (Chi) stood for /kʰ/ (later /x/; palatalized to [ç] in Modern Greek before front vowels). However, the Etruscans had taken over 'Χ' from western Greek, and it therefore stands for /ks/ in Etruscan and Latin.
The letter 'Χ' ~ 'Ψ' for /kʰ/ was a Greek addition to the alphabet, placed after the Semitic letters along with phi 'Φ' for /pʰ/.
In English orthography, ⟨x⟩ is typically pronounced as the voiceless consonant cluster when it follows the stressed vowel (e.g. ox), and the voiced consonant when it precedes the stressed vowel (e.g. exam). It is also pronounced when it precedes a silent ⟨h⟩ and a stressed vowel (e.g. exhaust). Before ⟨i⟩ or ⟨u⟩, it can be pronounced or (e.g. sexual and luxury); these result from earlier and . It also makes the sound in words ending in -xion (typically used only in British-based spellings of the language; American spellings tend to use -ction). When ⟨x⟩ ends a word, it is always (e.g. fax), except in loan words such as faux (see French, below).
There are very few English words that start with ⟨x⟩ (the fewest of any letter). When ⟨x⟩ does start a word, it is usually pronounced (e.g. xylophone, xenophobia, and xanthan); in rare recent loanwords or foreign proper names, it can also be pronounced (e.g. the obsolete Vietnamese monetary unit xu) or (e.g. Chinese names starting with Xi like Xiaomi or Xinjiang). Many of the words that start with ⟨x⟩ are of Greek origin, or standardized trademarks (Xerox) or acronyms (XC). In abbreviations, it can represent "trans-" (e.g. XMIT for transmit, XFER for transfer), "cross-" (e.g. X-ing for crossing, XREF for cross-reference), "Christ-" as shorthand for the labarum (e.g. Xmas for Christmas, Xian for Christian), the "crys-" in crystal (XTAL), or various words starting with "ex-" (e.g. XL for extra large, XOR for exclusive-or).
In Latin, ⟨x⟩ stood for [ks]. In some languages, as a result of assorted phonetic changes, handwriting adaptations or simply spelling convention, ⟨x⟩ has other pronunciations:
Additionally, in languages for which the Latin alphabet has been adapted only recently, ⟨x⟩ has been used for various sounds, in some cases inspired by European usage, but in others, for consonants uncommon in Europe. For these no Latin letter stands out as an obvious choice, and since most of the various European pronunciations of ⟨x⟩ can be written by other means, the letter becomes available for more unusual sounds.
In mathematics, x is commonly used as the name for an independent variable or unknown value. The modern tradition of using x to represent an unknown was introduced by René Descartes in La Géométrie (1637). As a result of its use in algebra, X is often used to represent unknowns in other circumstances (e.g. X-rays, Generation X, The X-Files, and The Man from Planet X; see also Malcolm X).
In the Cartesian coordinate system, x is used to refer to the horizontal axis.
It can be used as an abbreviation for 'between' in the context of historical dating; e.g., '1483 x 1485'.
Maps and other images sometimes use an X to label a specific location, leading to the expression "X marks the spot".
In art or fashion, the use of X indicates a collaboration by two or more artists, e.g. Aaron Koblin x Takashi Kawashima. This application, which originated in Japan, now extends to other kinds of collaboration outside the art world. This usage mimics the use of a similar mark in denoting botanical hybrids, for which scientifically the multiplication × is used, but informally a lowercase "x" is also used.Also for encodings based on ASCII, including the DOS, Windows, ISO-8859 and Macintosh families of encodings.
In the C programming language, "x" preceded by zero (as in 0x or 0X) is used to denote hexadecimal literal values.
X is commonly used as a prefix term in nouns related to the X Window System and Unix