Dental Fricative

PE19/71 (3.ii)

th became the interdental spirant [þ], more or less identical with the sound of th in English thin.

LotR AppE

TH represents the voiceless th of English in thin cloth.

VT41/7-8

This in Quenya was earlier, as it remained in Vanyarin, a dental spirant (made with the tongue-tip behind the back of the upper front teeth, which makes the passage to s easier). […] Why the Ñoldor did not then proceed to adopt the interdental þ and so clarify the distinction between þ and s remains uncertain.