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?
Cluster /kʃ/
Hindi romanization occasionally uses <x> to transliterate the character <क्ष> /kʃ/.
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 then begat x as /x/ in many linguist-devised Latin orthographies, natlang or conlang.
- Conlang examples: Olaetian (current transliteration), Tirelat, Zharranh, Tilya, Ludireo, Wanya (Unicode romanization), Jayus
Further fricatives
Voiceless fricativ values are not uncommon for x either.
Voiceless dental fricative
- For some very good reason that has unfortunately been lost to time, the conlang Enzielu uses x for /θ/.
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 alveolar lateral fricative
- Modern romanizations of Batu use x for /ɬ/. Romanizations of other Isiko languages tend to use the digraph tl.
Voiceless postalveolar fricative
Portuguese and Old Spanish use x for /ʃ/, a regular sound change from Latin /ks/. 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/.
Voiceless retroflex fricative
A simialr case as well: while no natlang use is known, Rejistanian writes /ʂ/ as x in the transliteration into the Latin alphabet.
Voiceless pharyngeal fricative
Somali uses x for /ħ/.
- Did your conlang run out of letters, too?
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.
- This may not have been his idea, as Nambikwara does the same too.
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
Ejectivity
Rising tone
- Iu Mien (that's the "Mien" of Hmong-Mien languages)
