@@@ combine with final [θ] became [s] after another [θ], [ð].

In Sindarin words where there are repeated dental spirants, one of them tends to dissimilate away from the other. The clearest example is úthaes, which Tolkien said was the result of dissimilation in úthaeth (VT44/30). A deleted variant úsaeth indicates that dissimilation of the first spirant away from the second is also possible. Other potential examples include:

• Aranrúth “King’s Ire” rather than **Aradhrúth, where the usual sound change of [nr] to [ðr] was inhibited by the final spirant (S/201).
• Hadhod “Dwarf” rather than **Hadhodh, where the usual sound change of [d] to [ð] after vowels was inhibited by the preceding spirant (WJ/388).
• [N.] Hadhafang “Throng-cleaver” the result of dissimilation of older †Haðathang, where th became f; Tolkien also considered Havathang (Ety/STAG).
• sammath “chambers” could also be the class plural of [N.] tham “hall” with dissimilation of its initial consonant (LotR/942).

Hat tip to Elaran and Vyacheslav Stepanov for providing these last three examples.