As used in a variety of places (real language: Icelandic; English -- but both are spelled the same 'th' there: note: 'THIS vs. THINK', 'THESE vs. THICK' and similar; and IPA symbols), the first (sometimes called 'eth' or 'ed' using that first symbol) and the second (sometimes called 'thorn' for the sound it starts with) are the voiced and unvoiced representations, respectively, of this one consonant.
If 'voiced' and 'unvoiced' doesn't make any sense to you now, you'll want to look both those up in terms of 'consonants' on the Internet.
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As used in a variety of places (real language: Icelandic; English -- but both are spelled the same 'th' there: note: 'THIS vs. THINK', 'THESE vs. THICK' and similar; and IPA symbols), the first (sometimes called 'eth' or 'ed' using that first symbol) and the second (sometimes called 'thorn' for the sound it starts with) are the voiced and unvoiced representations, respectively, of this one consonant.
If 'voiced' and 'unvoiced' doesn't make any sense to you now, you'll want to look both those up in terms of 'consonants' on the Internet.
It depends on the language and the period. In some contexts, they're pronounced the same (orthographic variants) and in some they distinguish voicing.