Voiced dental fricative

This is the interdental fricative which is voiced, the symbol for it being /ð/ (pronounced eth or /ɛð/).

English

Anglo-Saxon

Like all other fricatives in Anglo-Saxon, the symbols Þ,þ and Đ,ð were used for both intervocalic sounds. When they were intervocalic they were usually voiced (unless geminated) and thusly became /ð/. Many modern English words with the voiced /ð/ come from this era.

Middle English

In many cases, this still used the symbols Þ,þ and Đ,ð, though they were in a quick decline. In their place th came. Some wrote them as t or d, but many used th. In some cases there were interdental t̪ and d̪, which would later develop into the interdental fricatives. An example would be moder from Geoffry Chaucer. Later the interdental stop would become a fricative, and become mother, not moser or mozer indicating that the stop was interdental and not alveolar.

Modern English

Modern English uses the combination th to indicate both interdental fricatives. Some dialects replace the interdentals with dental stops (Irish English), labio-dental fricatives (Cockney and Philadelphian English), or alveolar fricatives (Pennsylvanian Dutch English). Although not the definite rule, most words in English which are written with an intervocalic th are voiced, such as bath /bæθ/ (voiceless) vs. bathe /beð/ (voiced). There are many words in Modern English with initial voiced interdental fricatives, such as the /ðə/, though /ðow/, then /ðɛn/, there /ðɛɹ/, that /ðæt/ etc. A minimal pair for the voiced and voiceless forms of the interdental fricatives are thy /ðaj/ (voiced), and thigh /θaj/ voiceless

Old Norse

Old Norse also had this sound. It was written in the Latin alphabet with the same two letters as it had been written with in the Anglo-Saxon alphabet, Þ,þ and Đ,ð. Many cases of Old Norse involved similar intervocalic fricatives becoming voiced, so it is likely that the symbols were used interchangeably. The letter þ, thorn, comes from the Runic alphabet.

Icelandic

Icelandic, unlike its Scandinavian cousins in Denmark, Norway, and Sweden, did not loose the interdental fricative sounds. The symbols Þ,þ and Đ,ð are still used today on the Icelandic keyboard.

Spanish

Spanish does not naturally have the interdental voiced fricative, but as an allophone. The letter d in Spanish is an interdental stop, /d̪/. When this sound is between, or in some cases after, a vowel, then it becomes /ð/. Examples include nada /naða/, todos /toðos/, and unidos /uniðos/.