A noun for a vaulted or arched roof, as well as chambers with such a roof, both constructed and natural. Tolkien’s most complete description of this word appeared in the Quendi and Eldar essay:
> S rond, Q rondo are from ✱rono “arch over, roof in”. This could be applied both to natural and to artificial structures, but its view was always from below and from the inside ... CE ✱rondo meant “a vaulted or arched roof, as seen from below (and usually not visible from outside)”, or “a (large) hall or chamber so roofed”. It was still often applied pictorially to the heavens after the Elves had obtained much greater knowledge of star-lore. Cf. the name Elrond “Star-dome“ (WJ/414).
Thus this word was sometimes applied (metaphorically) to the dome of heaven, though that last use may be limited to Sindarin.
Conceptual Development: The earliest precursor to this name seems to be ᴱQ. ronda appearing in the Early Noldorin Dictionary of the 1920s as a cognate of ᴱN. gronn “cavern” (PE13/162). ᴹQ. rondo “cavern” appeared in the Declension of Nouns from the early 1930s (PE21/8). Tolkien gave ᴹQ. rondo “roof, cave” in The Etymologies of the 1930s from the root ᴹ√ROD of the same meaning (Ety/ROD); in The Etymologies as published in The Lost Road Christopher Tolkien gave the gloss “cave” (LR/384), but in their Addenda and Corrigenda to the Etymologies, Carl Hostetter and Patrick Wynne corrected this to “roof, cave” (EtyAC/ROD).
Tolkien mentioned rondo “cave” in passing within rough notes on Felagund and related names from Notes on Names (NN) from 1957, but this entire block of notes was marked through (PE17/117-118). In addition to the aforementioned paragraph from the 1959-60 Quendi and Eldar essay given above, Tolkien gave rondo the gloss “vaulted hall” in a discussion of the strengthening of primitive nasals and stops (VT39/9). In this essay, Tolkien seems to have kept the basic form and meaning from The Etymologies of the 1930s, but deriving it instead from a new root √RON “arch over”.
Neo-Eldarin: For purposes of Neo-Eldarin, I prefer the earlier root form ᴹ√ROD as this lets us retain more of the 1930s forms.
rondo noun "a vaulted or arched roof, as seen from below" (and usually not visible from outside); "a (large) hall or chamber so roofed", "vaulted hall" _(WJ:414; VT39:9; in the Etymologies, stem ROD, the gloss is simply "cave" or "roof"; see VT46:12 for the latter gloss)_. Cf. *Elerondo.