Voiceless bidental fricative
The voiceless bidental fricative is a rare consonantal sound used in some languages. The only natural language known to use it is the Shapsug dialect of Adyghe. It is also used for a geminate voiceless glottal fricative (so phonemically ) in the original version of the constructed language Ithkuil, its offshoot Ilaksh, and the new version of Ithkuil as one of two allophones.

Features 

Features of the voiceless bidental fricative:

 Its place of articulation is bidental, which means it is articulated with the lower and upper teeth pressed together.

Occurrence