Q is a Latin letter ultimately descending from the Phoenician qoppa.
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?
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.
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/.
Albanian uses q for a voiceless palatal stop, /c/.
As a designation for conlangs
The following conlangs are or have been designated as Q: