The thorn. Concocted by scholarly
monks in the latter days of the
Roman occupation of Britain to represent the English TH sound, which was unknown in
Latin. Sometimes written as a kind of Y in
superscript, which is the origin of "ye," as in "Ye Olde Pub." This was never pronounced, and never meant to be pronounced, as it's (apparently) spelled--the Y was supposed to be a
thorn representing TH. But such is the memory of a people that its Y-like form has come to be commonly accepted as meaning that those weird old English-speakers said ye instead of the.
Frequently encountered by scholars of Anglo-Saxon literature when reading about Sir Gawain or the fight at Badon Hill.