A noun for “throne” in the phrase nai tiruvantes i hárar mahalmassen mi Númen “in the keeping of those who sit upon thrones of the West” (UT/305, 317). In the Quendi and Eldar essay of 1959-60, Tolkien said that mahalma was derived from Valarin maχallām of the same meaning and was “properly one of the seats of the Valar” (WJ/399). As such, this word is unlikely to be used for an ordinary “throne”, which instead would be tarhanwa.
Quenya
tarhanwa
noun. throne [high seat]
tarhanwa
noun. throne, (lit.) high seat
arcanwa
noun. throne [high seat]
mahalma
noun. throne
mahalma
throne
mahalma noun "throne", nominative pl. mahalmar "thrones" and locative pl. mahalmassen in CO. Adopted and adapted from Valarin (WJ:399)
A noun for “throne” in Late Notes on Verb Structure (LVS) written in 1969, a combination of tar- “high” and hanwa “seat” (PE22/148). Tolkien previously considered a form arkanwar which also seems to mean “✱thrones” (PE22/147 and note #10), where the initial element was ar(a)- “noble, high”.