The word sammath “chambers” appeared in the name Sammath Naur “Chambers of Fire” for the cavernous chambers in the interior of Mount Doom (LotR/942). It appears to be the class-plural of an otherwise unattested noun *sam “chamber”.

Conceptual Development: Earlier forms of this word were G. tham “chamber, room” from the Gnomish Lexicon of the 1910s (GL/72) and N. tham “hall” from The Etymologies of the 1930s under the root ᴹ√STAB (Ety/STAB). The Gnomish word was cognate to ᴱQ. sambe “room, chamber” in the Qenya Lexicon of the 1910s under the early root ᴱ√SAMA (QL/81), though the Gnomish form tham indicates the actual form of the root was (or became) *ᴱ√ÞAMA. Likewise the Noldorin word in the 1930s was cognate to ᴹQ. sambe “room, chamber” but with a distinct gloss = “hall” (Ety/STAB). The form sammath from the 1950s may indicate another revision of the root to √SAB or √SAM, though both of these conflict with other roots from the 1950s and 60s: √SAB “believe” (PE22/158) and √SAM “to have” (PE17/183).

Alternately, sammath could in fact be based on [N.] tham, with the initial consonant dissimilating away from the final -th, a phenomenon also seen in úthaes “temptation”. Hat-tip to Elaran for pointing this out to me.

Neo-Sindarin: I now prefer this final explanation of sammath as an abnormal class-plural of tham, and so would use tham for both “hall” and “chamber” in the singular, but would make sammath its class plural.