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In linguistics, specifically articulatory phonetics, tongue shape describes the shape that the tongue assumes when it makes a sound. Because the sibilant sounds have such a high perceptual prominence, tongue shape is particularly important; small changes in tongue shape are easily audible and can be used to produce different speech sounds, even within a given language.
For non-sibilant sounds, the relevant variations in tongue shape can be adequately described by the concept of secondary articulation, in particular palatalization (raising of the middle of the tongue), velarization (raising of the back of the tongue) and pharyngealization (retracting of the root of the tongue). Usually, only one secondary articulation can occur for a given sound.
In addition, the acoustic quality of velarization and pharyngealization is very similar so no language contrasts the two.
The following varieties of tongue shapes are defined for sibilants, from sharpest and highest-pitched to dullest and lowest-pitched:
The last three types of sounds are often known as "hushing" sounds because of their quality, as opposed to the "hissing" grooved sounds. Palatalization is an inherent part of the definition of the above varieties and cannot normally be varied independently.