Clicks are stops produced with two articulatory closures in the oral cavity. The pocket of air enclosed between the two closures is rarefied by a "sucking" action of the tongue. The release of the more forward closure produces a loud and extremely salient noise. This so-called velaric airstream mechanism is always ingressive (the air is sucked in) and can only be used for stops and affricates. Clicks are inherently stop-like or affricate-like depending on their place of articulation: clicks involving an alveolar or palatal closure are acoustically like plain stops, while bilabial, dental and lateral ones sound more like affricates.

Clicks are in all the Khoisan languages of southern Africa and in the neighbouring Nguni languages (Zulu, Xhosa, etc.) of the Bantu family, which borrowed them from Khoisan (there are some 80 languages in both groups). Clicks also occur in Sandawe and Hadza, two languages in Tanzania (believed by some to be distant branches of Khoisan), Sesotho, spoken in South Africa and Lesotho, and in Dahalo , a South Cushitic language spoken in Kenya. The only non-African language known to employ clicks as regular speech sounds is Damin, an "alternative code" used by speakers of Lardil (Australia) -- actually an elaborate kind of language game. The click in Damin is actually a 'reverse click', formed in an identical way to that described above, but involving an outward (egressive) movement of air, not an inward or ingressive movement, as in the African clicks. Of course "tut-tut" or "gee-up" noises can be used as meaningful interjections worldwide.

As noted above, clicks necessarily involve two closures: an anterior one which is regarded as primary and determines the click's place of articulation, and a posterior one which is typically velar or (less commonly) uvular. This posterior "accompaniment" can be transcribed as a velar or uvular oral or nasal. It's quite easy to pronounce a nasalised click if you realise that while maintaining the double oral closure you're free to breathe through the nose.

Since there are numerous (some of them really daunting) combinations of elements making up a click accompaniment, there are more than 100 ways of beginning a word with a click. These include a velar stop for basic clicks, a voiced velar stop, an aspirated one, a nasal one, a velar and glottal stop, a velar affricate, an ejective velar affricate, and others, as well. This means that pentagraphs like gk!x' are possible. The size of Khoisan click-phoneme systems ranges from 20 to as many as 83. In the latter language about 70% of words begin with a click; with the exception of Sandawe and Hadza, click languages permit only word-initial and word-medial clicks, never word-final.

The five clicks specified in the International Phonetic Alphabet (IPA) are the bilabial click , the dental click ǀ, the alveolar lateral click ǁ, the palatal click ǂ, and the postalveolar click ǃ. While the SAMPA encoding for IPA into ASCII doesn't have symbols for transcribing clicks, the proposed X-SAMPA standard does: O\, |\, |\|\, =\, and !. Some find the X-SAMPA symbols unsatisfactory, as the alveolar lateral click is represented by a doubled dental click symbol, rather than a dedicated character or character-plus-backslash, and suggest ||\, #\ or "\ instead. The Kirshenbaum system, an alternative system for representing IPA in ASCII, uses a different method to transcribe clicks. In this system, clicks are denoted by using the diacritic "!" after the homorganic voiceless stop. For example, /t!/ is the dental click and /p!/ is the bilabial click. Rather than using ASCII transcription schemes, the International Phonetic Association recommends the use of Unicode IPA symbols or IPA numbers which they have assigned to each symbol.

See also

• List of phonetics topics