DISCLAIMER: This article is preliminary research on the part of its author (Paul Strack) and does not necessarily reflect the opinions of the owner of this site. Since the source material is complex and its interpretation can be subjective, multiple conclusions are possible.
Sibilant mutation results from an ancient preceding s that was caused various mutation effects before being lost. The two best examples of sibilant mutation are the preposition o “about” and (possibly) the conjunction a “and”. The most complete description of sibilant mutation appears in a discussion of one of the etymologies of “and”:
a “and” (< as(a) = Q ar): in S. this a leaves the initials b, d, g, m, n, s unchanged; but changes p, t, c > f, th, ch; and aspirates vowel (a h-annon “and gate”), and r, l > rh, lh. In older Sindarin gw became chw < sw (for gw is only the S. initial form of basic [ancient] w-: as wath “shadow”, initial form gwath, i-wath “the shadow”, a chwath “and a shadow”); but in later S. on anal. of genuine original g (as in a galað “and a tree”) gw was left unaltered: a gwath “and a shadow” (PE17/41).
Thus this mutation has two affects. First, the h which is the normal medial development of s reappears before vowels: ah annon or a hannon “and a gate”. This is happens in the phrase Athrabeth Finrod ah Andreth “The Debate of Finrod and Andreth”, and a similar development is noted for [Noldorin] o “about” in The Etymologies of the 1930s:
OS- “round, about”. N o “about, concerning”, h before vowel as o Hedhil “concerning Elves” (Ety/OS).
The other affect is the voiceless stops become voiceless spirants (p, t, c → ph, th, ch) and voiced liquids r, l become voiceless rh, lh; this is the normal phonetic effect of a historical s preceding p, t, c, r, l. Though not mentioned by Tolkien, is likely that older initial ch would be preserved rather than becoming h as it does initially, so that initial h would “mutate” to ch.
Summary: I would rate this as a “mostly optional” mutation, in that its effects are minimal and are mentioned only in a single source. I personally would restore the h before vowels, but otherwise ignore any other mutational effects. If you do choose to use this mutation, its effects are:
- Initial voiceless stops (p, t, c) become voiceless spirants (ph, th, ch).
- h becomes ch.
- Voiced liquids l, r become voiceless lh, rh.