X

X is the 21st and final letter of the original Latin alphabet, adapted via Etruscan from Greek chi (Χ). Since its inception, it has gone on to become one of the "leftover" letters of the Latin alphabet, and has taken on a considerable array of phonetic values, however most commonly, one of /ʃ x ks/.

Velars

The three canonical values (and a few others) of x all involve velar consonants.

Aspirated voiceless velar stop

Or, /kʰ/. The original value of chi. Does anyone use it these days anymore?

Cluster /ks/

The original Latin value, as well as the value of chi in western varieties of the Greek alphabet. This is retained in several natlangs, including English, and has inspired several conlangs to prominently feature the cluster.

• Further examples here.

Cluster /gz/

The cluster has sometimes developed voice in English.

• And elsewhere?

Voiceless velar fricative

Spirantization got hang of chi in Greek by the Byzantine times, and the resulting fricativ value was handed down not only to Modern Greek, but also to the Cyrillic kha (Х), and doutlessly inspired by these, also to the IPA; which begat x as /x/ in many linguist-devised Latin orthographies, natlang or conlang.

• List examples here. Olaetian (current transliteration), Tirelat, Zharranh, Tilya, Ludireo

Further fricatives

Voiceless fricativ values are not uncommon for x either.

Voiced alveolar fricative

English pronounces initial x as /z/ in words such as xylophone. The original transliteration of the Olaetian alphabet used this letter for a laminal sound /z̻/ (while the apical sound /z̺/ is written z); current practice is to transcribe the laminal sound as ż.

Voiceless alveolar fricative

For some godforsaken reason, Vietnamese and Hmong use x for /s/.

• Please don't tell me you do this (at least, if you are not Vietnamese or Hmong yourself)

Voiceless postalveolar fricative

Portuguese and Old Spanish use x for /ʃ/. A large number of South American natlangs (and some elsewhere) get the same usage from them. Maltese also does this, though probably for unrelated reasons. Popular in conlangs as well.

• List examples here.

Voiceless alveolo-palatal fricative

Pinyin transcription of Mandarin, likely inspired by the previous, uses x for /ɕ/.

• Any other examples? Simik (a.k.a. Zirinka)

Voiceless pharyngeal fricative

Somali uses x for /ħ/.

• Did your conlang run out of letters, too?

Voiceless retroflex fricative

While no natlang use is known, Rejistanian writes this sound as <x> in the transliteration into the Latin alphabet.

Other consonants

And this is why we call it a "leftover" letter.

Voiced alveolar affricate

/dz/ is found in Albanian and Pashto. We are not sure whichever got it first, and if there is any connection anyway. There's just really no single fitting Latin letter for /dz/ once you've spent z on /z/, and at least this is better than making up some sort of a voiced c (oh, those wacky Turks…)

Alveolar ejective affricate

Kiowa, spoken not anywhere near Kiev but in Oklahoma, together with other unintuitiv letter assignments, uses x for /tsʼ/.

Alveolar ejective stop

In the East Cushitic Oromo, x = /tʼ/.

Alveolar implosive

The fellow East Cushitic Afar goes by x = /ɗ/. We suspect this may be cognate with the previous.

Glottal stop

As if Pirahã wasn't weird enuff yet, Daniel Everett had to pick x for /ʔ/, even with q unused.

Postalveolar click

/ǃ/ and variations thereof are the standard usage of x in Nguni languages such as Zulu.

Oh, but we're not done yet…

(*supervillaneous mad cackle*)

Schwa

• Sasxsek

Rising tone

• Iu Mien (that's the "Mien" of Hmong-Mien languages)

General all-purpose diacritic

• ASCIIfication of Esperanto and gjâ-zym-byn