OS. [ē], [ō] became [ī], [ū]; [ē|ō] > [ī|ū]
In both (Old) Sindarin and (Old) Noldorin, the long vowels [ē] and [ō] became [ī] and [ū]. Where [ē] and [ō] occur in Sindarin and Noldorin words, they are generally the result of the lengthening of short vowels in monosyllables. The same sound changes also occurred in the earliest versions of the language (Gnomish and Early Noldorin), and similar changes occurred in Welsh (WGCH/§59i), though with Welsh [ō] > [ū] only in final syllables. Tolkien described these changes several times over his life:
ê gave î ... ô [gave] û (Gnome Vowels from the 1910s, PE15/13).
In the earliest recorded Noldorin, [ē, ǣ, ā, ǭ, ō] remained distinct though changed to [ī, ei, ǭ, ou, ū] (first version Tengewesta Qenderinwa [TQ1] from the 1930s, PE18/46).
In the earliest Noldorin ē, ę̄, ā, ǭ, ō appear as ī, ę̄, ǭ, ǭ, ū (second version Tengewesta Qenderinwa [TQ2] circa 1950, PE18/96).
C.E. ei, oi > [S.] ē > ī ... ō > ū (diphthong chart associated with Tolkien’s discussion of Hands, Fingers and Numerals from the late 1960s, VT48/7).
Examples are plentiful. Here are a few clear examples from each major period:
- ᴱ✶tē > G. tî “straight” (GL/71).
- ᴱ✶t’lḗpe > ᴱN. tlî “butter” (PE13/154).
- ᴹ✶nēthē > N. nîth “youth” (Ety/NETH).
- ✶dērā > S. dîr “hard, difficult” (PE17/154).
- ᴱ✶Noronōr- > G. Dronūrin “Messenger of the Gods” (GL/31).
- ᴱ✶kópa > ᴱN. cû “bay, cove” (PE13/141).
- ᴹ✶b’rōnā > ON. brūna > N. brûn “old” (Ety/BORÓN).
- ✶χīþilōmē > hithlũṽ > S. Hithlũ “Hithlum” (PE17/133).
Based on the last two examples, it seems this sound change took place in Old Noldorin/Old Sindarin, but relatively late, after aspirates became voiceless spirants. The timing of this sound change is harder to determine in the earlier conceptual periods.
Conceptual Development: This seems to be an exceptionally stable and well documented phonetic rule, established very early and maintained thereafter.