The interdental fricative is a relatively rare sound in the languages of the world. Most languages lack the sound. In English it is represented by the letters th. The English interdental fricative can be voiced as in the or voiceless as in through.
Languages besides English which have the voiced interdental fricative include Albanian, Catalan, Danish, Greek, Icelandic, Spanish and Welsh. However, the Danish and Icelandic fricatives are classified as alveolar.
The voiceless interdental fricative is less common than the voiced counterpart. It's found in Burmese, Greek and Icelandic. The Icelandic pronunciation is alveolar. In Spanish it's only found in the dialect spoken in Spain and in Arabic it's only found in a few dialects such as the one spoken in Iraq.
The interdental fricative is a marked consonant. English is rare among the languages of the world because it has both the voiced and voiceless counterparts. This is also true for European Spanish, Greek and Icelandic. The place of articulation of the consonant can also be dental or alveolar.