Sulcalization (from Latin: sulcus 'groove'), also called internal rounding, in phonetics, is the pronunciation of a sound, typically a sibilant consonant, such as English /s/ and /z/, with a deep groove running along the back of the tongue that focuses the airstream on the teeth, producing a more intense sound.
This is accomplished by raising the sides of the back of the tongue ("lateral contraction") and leaving a hollow along the mid-line.
/θ/ has also been reported to show tongue grooving in English, despite being a non-sibilant fricative (Stone & Lundberg 1996).
For example, close vowels triggered this effect in Japanese, where historic *tu and *ti have become [tsu] and [tɕi], respectively.
Accordingly, the realization of the /ə/-element of the centring diphthongs /ɪə̯/, /ʊə̯/, /ɛə̯/ in words such as near, pure and scare, is interpreted as the product of a loss of sulcality (Erickson 2003: 197).