In English, the digraph ⟨th⟩ represents in most cases one of two different phonemes: the voiced dental fricative /ð/ (as in this) and the voiceless dental fricative /θ/ (thing).
More rarely, it can stand for /t/ (Thailand, Thomas) or the cluster /tθ/ (eighth).
The letters "TH" are used to represent the voiced and the voiceless dental fricatives (/ð/ and /θ/, respectively).
These phonemes are two of, while common in the English language, but are among the rarest phonemes globally.
Old English had two separate letters for the "th" sound.
The first is written like this: þ.
It is called thorn. ð and Ð (eth): Old English scribes could also represent the "th" sound with the letter ð (the capital letter version looks like a capital D with a short horizontal line: Ð).