Sulcalization , in phonetics, is the pronunciation of a sound, typically a sibilant consonant, such as English and , 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 and leaving a hollow along the mid-line. It is not clear if all sibilants are grooved to at least some extent. Catford observed that the degree of tongue grooving differs between places of articulation as well as between languages; however, no language is known to contrast sibilants based purely on the presence or absence of tongue grooving.