Q

Q is a Latin letter ultimately descending from the Phoenician qoppa.

Back stops

Labialized voiceless velar stop

Latin adopted Q from Etruscan, where it represented /k/ before /u/. This motivated its use for Latin's /kʷ/. In classical Latin, this is usually rendered as QV.

• Conlang examples?

Voiceless uvular stop

In most modern Semitic languages, Proto-Semitic *q is a voiceless uvular stop, and hence q is used for this purpose in the transliteration of those languages, inspired by this in the IPA, and consequently also in countless modern Romanizations from Aleut to Yucatec. Some variations occur, such as /qʰ/ in Eyak.

• Conlang examples go here, etc.

Voiceless velar stop

Most of the westerly Romance languages (such as Spanish) have, subsequent to the palatalization of Latin /k/ before front vowels, also delabialized /kʷ/ in the same context. This has lead to qu (or, if you will, q with the u being silent) being used for /k/.

Those West Germanic languages that have loaned q also use it for /k/, similar to Classical Latin usually only an an allograph in a cluster /kw/ or /kv/.

Other uses

Pinyin transcription of Mandarin Chinese uses q for a voiceless aspirated palato-alveolar affricate, /tɕʰ/.

Visual similarity to ŋ has motivated the use of q for various velar nasal phonemes, eg. a prenasalized stop /ŋg/ in Fijian.

Zulu, Xhosa etc. use q for a postalveolar click, /ǃ/.

Afar uses q for /ʕ/.

anything else?