16th letter of the classical Roman alphabet, occurring in English only before a -u- that is followed by another vowel (with a few exceptions; see below), whether the -u- is sounded or not (pique). The letter is from the Phoenician equivalent of Hebrew koph, qoph, which was used for the deeper and more guttural of the two "k" sounds in Semitic. The letter existed in early Greek (where there was no such distinction), and called koppa, but it was little used and not alphabetized; it mainly served as a sign of number (90).
The connection with -u- began in Latin. Anglo-Saxon scribes at first adopted the habit, but later used spellings with cw- or cu-. The qu- pattern returned to English with the Normans and French after the Conquest and had displaced cw- by c. 1300.
In some spelling variants of late Middle English, quh- also took work from wh-, especially in Scottish and northern dialects, for example Gavin Douglas, Provost of St. Giles, in his vernacular "Aeneid" of 1513:
Lyk as the rois in June with hir sueit smell
The marygulde or dasy doith excell.
Quhy suld I than, with dull forhede and vane,
With ruide engine and barrand emptive brane,
With bad harsk speche and lewit barbour tong,
Presume to write quhar thi sueit bell is rong,
Or contirfait sa precious wourdis deir?
Scholars use -q- alone to transliterate Semitic koph or the equivalent in Turkish or Iranian (as in Quran, Qatar, Iraq). In Christian theology, Q has been used since 1901 to signify the hypothetical source of passages shared by Matthew and Luke but not in Mark; in this sense probably it is an abbreviation of German Quelle "source" (from Old High German quella, from the same Proto-Germanic source as Old English cwiella, cwylla"spring; well"). In Middle English accounts, it is an abbreviation of quadrans "farthing" (mid-15c.). In Roman personal names it is an abbreviation of Quintus.