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) stands for /kʰ/ (later /x/). 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/. (The variant 'Ψ' later replaced the digraph 'ΦΣ' for /ps/; omega was a later addition)..
In English orthography, x is typically pronounced as the voiceless consonant cluster /ks/ when it follows the stressed vowel, and the voiced consonant /ɡz/ when it precedes the stressed vowel (e.g. 'exam'), or when it precedes a silent 'h' and an accented vowel ('exhaust'). Before 'i' or 'u' it can also be pronounced /kʃ/ or /ɡʒ/ for example, in the words 'sexual' and 'luxury', respectively: these result from earlier /ksj/ and /ɡzj/. It also makes the sound /kʃ/ in words ending in -xion (typically used only in British-based spellings of the language; American spellings tend to use -ction). Word-final 'x' is always /ks/ (e.g. 'ax'/'axe') except in loan words such as 'faux' (see French, below).
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).
There are very few English words that start with X – the least amount of any letter. Many of the words that do start with X are either standardized trademarks (Xerox) or acronyms (XC). No words in the Basic English vocabulary begin with 'X', but it occurs in words beginning with other letters.
X is the third least common letter in English, with a frequency of about 0.15% in words. It is, however, more frequent than Q and Z.
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:
Basque: as a spelling for[ʃ]. Additionally there is the digraph 'tx' [tʃ].
Dutch: X usually represents [ks], except in the name of the island of Texel, which is pronounced Tessel. This is because of historical sound-changes in Dutch, where all -x- sounds have been replaced with -s- sounds. Words with an -x- in the Dutch language are nowadays usually loanwords.
In Norwegian, 'x' is generally pronounced [ks], but since the 19th century, there has been a tendency to spell it out as 'ks'; it may still be retained in personal names, though it is fairly rare, and occurs mostly in foreign words and SMS language. Usage in Danish, German and Finnish is similar.
French: at the ends of words, silent (or [z] in liaison if the next word starts with a vowel). Three exceptions are pronounced [s]: six ("six"), dix ("ten") and Bruxelles. It is pronounced [z] in sixième and dixième.
In Italian, 'x' is either pronounced [ks], as in extra, uxorio, xilofono, or [ɡz], as exogamia, when it is preceded by 'e' and followed by a vowel. In several related languages, notably Venetian, it represents the voiced sibilant [z]. It is also used, mainly amongst the young people, as a short written form for "per", meaning "for": for example, "x sempre" ("forever"). This because in Italian the multiplication sign (similar to 'x') is called "per". However, 'x' is only found in loanwords, as it is not part of the standard Italian alphabet; in most words with 'x', this letter may be replaced with 's' or 'ss' (with different pronunciation: xilofono/silofono, taxi/tassì) or, rarely, by 'cs' (with the same pronunciation: claxon/clacson).
Spanish: In Old Spanish, 'x' was pronounced [ʃ], as it is still currently in other Iberian Romance languages. Later, the sound evolved to a hard [x] sound. In modern Spanish, the [x] sound is generally spelled as the letters 'j' or 'g', though 'x' is still retained for some names (notably 'México', which alternates with 'Méjico'). Presently, 'x' represents the sound [s] (word-initially), or the consonant cluster [ks] (e.g. oxígeno, examen). Rarely, the 'x' can be pronounced [ʃ] as in Old Spanish in some proper nouns such as 'Raxel' (a variant of Rachel) and Uxmal.
In Galician (a language related to Portuguese and spoken in Northwestern Spain), and Leonese, used in Spain, 'x' is pronounced [ʃ] in most cases. In learned words, such as 'taxativo' (taxing), the 'x' is pronounced [ks]. However, Galician speakers tend to pronounce it [s], especially when it appears before plosives, such as in 'externo' (external).
In Catalan, 'x' has three sounds; the most common is [ʃ]; as in 'xarop' (syrup). Other sounds are: [ks]; 'fixar' (to fix), [ɡz]; 'examen'. In addition [ʃ] gets voiced to [ʒ] before voiced consonants; 'caixmir'. Catalan also has the digraph 'tx', pronounced [tʃ].
In Portuguese, 'x' has four main sounds; the most common is [ʃ], as in 'xícara' (cup). The other sounds are: [ks] as in 'fênix/fénix' (phoenix); [s], when preceded by E and followed by a consonant, as in 'contexto' ([ʃ] in European Portuguese), and in a small number of other words, such as 'próximo' (close/next); and (the rarest) [z], which occurs in the prefix 'ex' before a vowel, as in 'exagerado' (exaggerated). A rare fifth sound is [ɡz], coexisting with [z] and [ks] as acceptable pronunciations in exantema and in words with the Greek prefix 'hexa'.
In Venetian it represents the voiced alveolar sibilant [z] much like in Portuguese 'exagerado', English 'xylophone' or in the French 'sixième'. Examples from medieval texts include raxon (reason), prexon (prison), dexerto (desert), chaxa/caxa (home). Nowadays, the best-known word is xe (is/are). The most notable exception to this rule is the name Venexia[veˈnɛsja] in which 'x' has evolved from the initial voiced sibilant [z] to the present day voiceless sibilant.
In Maltese, 'x' is pronounced [ʃ] or, in some cases, [ʒ] (only in loanwords such as 'televixin', and not for all speakers).
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 started by René Descartes in La Géométrie (1637).
It may also be used to signify the multiplication operation when a more appropriate glyph is unavailable. In mathematical typesetting, 'x' meaning an algebraic variable is normally in italic type (), partly to avoid confusion with the multiplication symbol. In fonts containing both 'x' (the letter) and '×' (the multiplication sign), the two glyphs are dissimilar.
Other non-mathematical uses include:
As a result of its use in algebra, X is often used to represent unknowns in other circumstances (e.g. Person X, Place X, etc.; see also Malcolm X).
X-rays are so called because their discoverer did not know what they were.
X has been used as a namesake for a generation of humans: Generation X, commonly abbreviated to Gen X. It is the generation born after the baby boom ended, ranging from 1961 to 1981.
An X-shaped mark has traditionally been used by the illiterate in lieu of a signature, and is also used to indicate a signature line on forms.
X marks are used to indicate the concept of negation or incorrect, the opposite of a Tick (check mark). They are also used as a substitute for the check mark (most notably on election Ballot papers)
X is commonly used as a generic mark (selecting an item on a form, indicating a location on a map, etc.).
The common custom of placing Xs on envelopes, notes and at the bottom of letters to mean kisses dates back to the Middle Ages, when a Christian cross was drawn on documents or letters to mean sincerity, faith, and honesty.
Usually in art or fashion, the use of X indicates a collaboration with two or more artists.[clarification needed] The application extends to any other kinds of collaboration outside the art world.
In cartoons, a dead character's eyes are often drawn as Xs.
In mapping by the standards of the USGS, an x-type mark is used to denote the point referred to by an elevation marking on topographical maps.
Maps leading to hidden treasure often denote the treasure with an X. The expression "X marks the spot" is related to these treasure maps