Old English, Old Norse, and Icelandic alphabets, as well as some dialects of Middle English. It was also used in medieval Scandinavia, but was later replaced with the digraph th. The letter originated from the rune ᚦ in the Elder Fuþark, called thorn in the Anglo-Saxon and thorn or thurs ("giant") in the Scandinavian rune poems, its reconstructed Proto-Germanic name being Thurisaz.
It has the sound of either a voiceless dental fricative [θ], like th as in the English word thick, or a voiced dental fricative [ð], like th as in the English word the. Modern Icelandic usage excludes the latter, which is instead represented with the letter eth (Ð, ð), though it has a voiceless allophone [θ], which occurs in certain positions within a phrase.
In its typography, the thorn is one of the few characters in a Latin-derived alphabet whose modern lower-case form has greater height than the capital in its normal (roman), non-italic form.
When reading sixteenth, seventeenth and eighteenth century documents written in Scotland expect to come across a letter which is now defunct, and which, confusingly, looks like a y. This is the archaic letter thorn. It fell out of use because of the standardisation of letters by printers.
The thorn looks very like a y, and represented the sound th. In the image below, the word the starts with a thorn.
This is why today one can find signs saying things like Ye Olde Coffee Shoppe. This is a misrepresentation. People in the past did not say 'ye' they said 'the'; it's just that they had a separate letter, the thorn, which denoted the sound th. When transcribing a thorn, write th.
Short words beginning with a thorn were often abbreviated. For example the word in Figure 4 is that written with a thorn and a superscript t superscript to show that something is missing - in this case the letter a. It should be subscribed th[a]t
It is also possible to find thorn in the middle of common words as shown below with oth[e]r and broth[e]r.