When I did a bunch of digging into the Thorn character (Þ þ A Thorn in the Alphabet’s Side) I found out about Eth (Ð, ð). The eth character was found in a number of Northern European scripts during the Middle Ages. In my initial readings, it looked like eth was used for the voiced “th” sound in “this” and “that” and that thorn was used for the unvoiced “th” sound in words like “thick” and “thin”.
About this voiced and unvoiced stuff: “Voiced” means that your larynx is involved when you make a sound. “Unvoiced” means that your larynx is not involved when you make a sound. Your mouth is doing the same thing with the “th” sound for “this” and “thick” but your larynx is buzzing when making the “th” sound for “this”. Similar pairs of voiced/unvoiced sounds include v/f, b/p, and d/t.
When I first found out about eth, I was sorta excited (Yes, I know. Don’t bother telling me.) to find a hard and fast rule to distinguish the voiced and unvoiced “th” sounds. However, my excitement was premature. Further reading told me that in Old and Middle English eth was often used for the voiced “th” sound in “this” and “that” but not always. Sometimes, it was silent. Other times, it also included the unvoiced “th” sound in words like “thick” and “thin”.
Later, when reading about linguistics, I found that the IPA (International Phonetic Alphabet) uses eth for the voiced “th” sound and the Greek Theta (Θ, θ) symbol for the unvoiced “th” sound. This made me excited again to be able to symbolize the 2 sounds discreetly.
Life is good.