I’ve already written about eth (Ð, ð) and thorn (Þ þ). When I was researching those posts, I felt some frustration because it was tough to find concrete answers about when and where these symbols were used. When listening to episode 36 of the History of English podcast by Kevin Stroud (http://historyofenglishpodcast.com/2013/12/23/episode-36-finalizing-the-alphabet/), I got an explanation of why these answers were so vague.
In the early dark ages, folks were trying to adapt the Roman alphabet to write old English.
In the north of England, Irish missionaries were setting up monasteries and writing stuff. When they wrote Old English, they found that the Roman alphabet didn’t help them represent the theta sound. To get around this, they used the letter D to represent the theta sound figuring that it was closest. That’s not as strange as you’d first think. A number of accents will say “dese” and “doze” instead of “these” and “those”.
Since the letter D was already used for the D sound, they put a cross on D’s vertical bar to indicate that when the symbol was for theta instead.
In the south of England missionaries from the Roman church were setting up monasteries, and writing stuff. Their approach to representing the theta sound was to borrow the thorn symbol from the Anglo-Saxon runes.
So which was used where?
The eth was used more-or-less-mostly in the north of England and the thorn was used more-or-less-mostly in the south. There weren’t any clearly defined borders for these conventions and certainly no standard dictionaries or style manuals.
The French came along in 1066 and got rid of all the non-Roman letters.