The voiceless dental non-sibilant fricative is a type of consonantal sound used in some oral languages. It is familiar to English speakers as the 'th' in thing. Though rather rare as a phoneme in the world's inventory of languages, it is encountered in some of the most widespread and influential (see below). The symbol in the International Phonetic Alphabet that represents this sound is , and the equivalent X-SAMPA symbol is T. The IPA symbol is the Greek letter theta, which is used for this sound in post-classical Greek, and the sound is thus often referred to as "theta".