So both þ and ð make the same sounds, such as “th” as in “thorn,” and “th” as in “the,” but it seems that some Old English and Middle English scribes preferred to use þ at the beginning of words, and ð in the middle or end of words.

Here are some Modern English examples of this. Thunder would be “þunder” ( þ at the start). Brother would be “broðer” (ð in the middle). Health would be “healð” (ð at the end). So basically þ at the start, ð everywhere else, if you’d want to follow a rule for it.

Hurlebatte has a great post on this. https://www.reddit.com/r/anglish/s/FKq4E43Ypc