Thorn or þorn is a letter (Þ or þ) that was used in several extinct alphabets, including Old English, and is used currently in Icelandic.
In most instances it's been replaced by th.
Þ usually sounds like the th in thin, but sometimes it's sounded like the th in this.
Early Modern English found thorn quite useful, especially in the mediaeval version of text, where þ followed by an e, t, or u was a common abbreviation for the, that or thou.
This beautiful and unassuming letter has been causing all sorts of difficulties, though. People have been asking if, as it's not used in many languages, can't it be got rid of altogether?
Does it count as a real letter at all?
If we must have it, where does it go in the alphabet?
Well, I'm glad to report that þ has a passionate fan group, so it doesn't look as if it's going to go away any time soon.
And as its history can be traced back a lot further than modern fripperies like the letter G (some people say þ has its roots in the Etruscan or Phoenician languages) no one can really say it isn't a proper letter.
But where should it go in the alphabet? Well, it's been put between T and U, (because it sounds like th); after TH and before TL; mixed up with TH; mixed with P (because it does look a bit like a P); written as Y and mixed with the other Y s; written as either TH or T and bunged in there; or put in after Z.
The Icelanders put þ after Z. Adding new letters to the end of the alphabet has quite often happened with new letters, so this does seem a reasonable solution.
In any rate, it's one that's been going since the 1000s, and it hasn't done any harm so far. So why not.
Thing of beauty to appreciate today: þ. The letter was first seen as the rune Þ in the Elder Futhark. It's called thurs or giant in the Scandinavian rune poems, and its Proto-Germanic name may have been thurisaz.