Þorn

Everyone's familiar with those old fashioned signs like 'Ye Olde Pub' or 'Ye Local Tavern.' The thing is, a lot of people are pronouncing it wrong. Because that Y isn't actually a Y, it's the letter Þ. Called Thorn, it gives us the same two sounds as the letters T and H put together. So for instance, instead of spelling it Thorn, it would actually be Þorn. Until about the 14th century, Thorn was a very integral part of the alphabet, since the sounds it makes appear in quite a few words. So what happened?

When France got a hold of the alphabet, they started adding letters that didn't need to be there, and so needed to take out some of the other ones. So they got rid of Thorn. Now, why did it get replaced by the letter Y? Because in traditional writing, the letter's curve was a bit higher up, and a bit straighter, so it looked like the letter Y with an extra line in the middle. Which is why a lot of the Y's on those signs you see look so stylistic. The letter Thorn, when used, actually made words more concise, in that they weren't really words. They were really more like abbreviations.

For instance, the word "The" was þe
And þe word that, was þt
This was þs and they was þy. Much quicker, much easier to write. Unfortunately, if you want to see the letter Thorn still used commonly, you're going to have to move to Iceland.