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⟨þ⟩ and ⟨ð⟩ (and as uppercase forms, ⟨Þ⟩ and ⟨Ð⟩) can be read as ⟨th⟩. ⟨þ⟩ and ⟨ð⟩ are not phonetically interchangable.
I use ⟨þ⟩ to represent a voiceless dental fricative—/θ/ or ‘th’ from „thirsty”. I use ⟨ð⟩ to represent a voiced dental fricative—/ð/ or ‘th’ from „thy”.