Voiceless dental fricative
This is the interdental fricative which is voiced, the symbol for it being /θ/ (pronounced theta or /θetə/ in English).
Like all other fricatives in Anglo-Saxon, the symbols Þ,þ and Đ,ð were used for both interdental sounds. When they were pre- or post-vocalically, or they were geminated (doubled) they were usually voiceless and thusly were /θ/. This means when these letters were initial, final, or doubled, it would always be this sound. This means, Many modern English words with the voiced /θ/ come from this era.
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 Geoffrey 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 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 voicless interdental fricatives, such as think /θɪŋk/, heath /hiθ/, thorn /θoɹn/, thaw /θɔ/, health /hɛlθ/ etc. The combination thr is always with the voiceless form leading to /θɹ/. Examples include three /θɹi/, through /θɹu/, and threat /θɹɛt/. A minimal pair for the voiced and voiceless forms of the interdental fricatives are thy /ðaj/ (voiced), and thigh /θaj/ voiceless.
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 the voiceless interdentals to be the default form of those sounds, so it is likely that the symbols were used interchangeably. The letter þ, thorn, comes from the Runic alphabet.
Icelandic, unlike its Scandinavian cousins in Denmark, Norway, and Sweden, did not lose the interdental fricative sounds. The symbols Þ,þ and Đ,ð are still used today on the Icelandic keyboard.
In Old High German, before the High German Consonant Shift, /θ/ was in the language. However, after the sound shift it and its voiceless counterpart swiftly became /d/.
Spanish does not naturally have the interdental voiceless fricative, but as a phoneme. The letters c (before i or e) and z in Spanish are an interdental voiceless fricatives, /θ/. This creates minimal pairs between words with s and those with c or z. However, this is only really restricted to Castilian Spanish in Spain, and not even the entire country of Spain. In the Americas, the sound is more commonly pronounced /s/. Examples include cielo /θielo/, piezas /pieθas/, and cero /θero/. Note: This is a legend that the Spanish /θ/ came from the kings and queens of Spain that were inbreeding. This is a myth, the development of this sound came from an originally retroflex /ʂ/;
The Greek letter for the interdental voiced fricative is Θ,θ also known as theta. In Ancient Greek this letter was /tʰ/, but in modern Greek it is /θ/.
The combination of th produces the /θ/ sound in Welsh.
Classical Arabic has /ð/ as the letter ث.