Editing Click consonant (section)

===The back-vowel constraint===
[[Image:Clicks and the BVC.svg|thumb|The shape of the tongue in Nama when articulating an alveolar click (blue) and a palatal click (red) [throat to the right]. The articulation of the vowel {{IPA|[i]}} is slightly forward of the red line, with its peak coinciding with the dip of the blue line.]]
In several languages, including [[Nama language|Nama]] and [[Juǀʼhoan dialect|Juǀʼhoan]], the alveolar click types {{IPA|[ǃ]}} and {{IPA|[ǁ]}} only occur, or preferentially occur, before [[back vowel]]s, whereas the dental and palatal clicks occur before any vowel. The effect is most noticeable with the high front vowel {{IPA|[i]}}. In Nama, for example, the diphthong {{IPA|[əi]}} is common but {{IPA|[i]}} is rare after alveolar clicks, whereas the opposite is true after dental and palatal clicks. This is a common effect of [[uvular consonant|uvular]] or uvularised consonants on vowels in both click and non-click languages. In [[Taa language|Taa]], for example, the back-vowel constraint is triggered by both alveolar clicks and uvular stops, but not by palatal clicks or velar stops: sequences such as {{IPA|*/ǃi/}} and {{IPA|*/qi/}} are rare to non-existent, whereas sequences such as {{IPA|/ǂi/}} and {{IPA|/ki/}} are common. The back-vowel constraint is also triggered by labial clicks, though not by labial stops. Clicks subject to this constraint involve a sharp [[retracted tongue root|retraction]] of the tongue during release.

{|class="wikitable"
! !!Abrupt <br />release!!Noisy <br />release
|-
|ballistic tongue retraction<br />& back-vowel constraint
|align=center|{{IPA|ǃ}}||align=center|{{IPA|ǁ, ʘ}}
|-
|no retraction, no constraint
|align=center|{{IPA|ǂ}}||align=center|{{IPA|ǀ}}
|}

Miller and colleagues (2003) used [[ultrasound imaging]] to show that the rear articulation of the alveolar clicks ({{IPA|[ǃ]}}) in Nama is substantially different from that of palatal and dental clicks. Specifically, the shape of the body of the tongue in palatal clicks is very similar to that of the vowel {{IPA|[i]}}, and involves the same tongue muscles, so that sequences such as {{IPA|[ǂi]}} involved a simple and quick transition. The rear articulation of the alveolar clicks, however, is several centimetres further back, and involves a different set of muscles in the uvular region. The part of the tongue required to approach the palate for the vowel {{IPA|[i]}} is deeply retracted in {{IPA|[ǃ]}}, as it lies at the bottom of the air pocket used to create the vacuum required for click airstream. This makes the transition required for {{IPA|[ǃi]}} much more complex and the timing more difficult than the shallower and more forward tongue position of the palatal clicks. Consequently, {{IPA|[ǃi]}} takes 50 [[millisecond|ms]] longer to pronounce than {{IPA|[ǂi]}}, the same amount of time required to pronounce {{IPA|[ǃəi]}}.

Languages do not all behave alike. In [[Nǁng language|Nǀuu]], the simple clicks {{IPA|/ʘ, ǃ, ǁ/}} trigger the {{IPA|[əi]}} and {{IPA|[æ]}} allophones of {{IPA|/i/}} and {{IPA|/e/}}, whereas {{IPA|/ǀ, ǂ/}} do not. All of the affricated contour clicks, such as {{IPA|/ǂ͡χ/}}, do as well, as do the uvular stops {{IPA|/q, χ/}}. However, the occlusive contour clicks pattern like the simple clicks, and {{IPA|/ǂ͡q/}} does not trigger the back-vowel constraint. This is because they involve tongue-root raising rather than [[Advanced and retracted tongue root|tongue-root retraction]] in the uvular-pharyngeal region. However, in [[Gǀwi dialect|Gǀwi]], which is otherwise largely similar, both {{IPA|/ǂ͡q/}} and {{IPA|/ǂ͡χ/}} trigger the back-vowel constraint (Miller 2009).