OS. [ē], [ō] became [ī], [ū]; [ē|ō] > [ī|ū]

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:

Based on the last two examples, it seems this sound change took place in Old Sindarin, but relatively late, after aspirates became voiceless spirants.

Conceptual Development: This seems to be an exceptionally stable and well documented phonetic rule, established very early and maintained thereafter. In Old Noldorin, however, it seems this change took place earlier than in Old Sindarin, before [ɣ] vanished. For example:

If the ʒ [ɣ] vanished first, the result would have been [teɣē] > [tē] > [tī].

Reference ✧ PE18/96

Order (03500)

After 01700 long final vowels were shortened kyelepē > S. celeb Let/426
After 01800 [z] vanished before [d] lengthening preceding vowel reddā > rezdā > S. rîdh PE19/91
After 02200 [ei], [ou] became [ī], [ū] ei > ē > OS. ī
ou > ō > OS. ū
VT48/7
VT48/7
After 02500 aspirates became voiceless spirants χīþilōmē > hithlũṽ > S. Hithlũ PE17/133
Before 03600 [j] vanished before [i], [ui] ᴹ✶at-yēn-ar > N. edinar Ety/YEN

Phonetic Rule Elements

[ē] > [ī] ✧ PE18/96 (ē > ī); PE19/91 (ē > ī)
[ō] > [ū] ✧ PE18/96 (ō > ū); PE19/91 (ō > ū)

Phonetic Rule Examples

dē > dī ē > ī > S. di ✧ PE17/95
dēra > dīra ē > ī dērā > S. dîr ✧ PE17/154
edēwe > edīwe ē > ī DEWE > S. edíw ✧ PE17/151
ēde > īde ē > ī Ezdē > S. Íð ✧ PE19/92
ēde > īde ē > ī ezdē > S. îdh ✧ PE19/91
ēde > īde ē > ī ezdē > S. îdh ✧ WJ/403
kēwa > kīwa ē > ī kēwā > S. cîw ✧ VT48/8
mēlambar > mīlambar ē > ī mēlā̆-mbar > S. milbar ✧ PE17/109
mēlambar > mīlambar ē > ī mēlā̆-mbar > mîl-mbar > S. milbar ✧ PE17/164
nēna > nīna ē > ī nēnā > S. nîn ✧ PE17/52
nēnatalma > nīnatalma ē > ī nēn-talma > S. nindalf ✧ PE17/167
nēθa > nīθa ē > ī nēthā > S. nîth ✧ VT47/14
rēda > rīda ē > ī reddā > rezdā > S. rîdh ✧ PE19/91
rēne > rīne ē > ī rēnē > S. rîn ✧ PM/372
tēw > tīw ē > ī teŋmā > teŋwā > tēwa > S. tîw ✧ PE22/149
-wē > -wī ē > ī > S. -wi ✧ PE17/189
xēru > xīru ē > ī kherū > S. hir ✧ Let/282
drōb > drūb ō > ū D)ROB > S. drû ✧ PE17/99
dūme > dūme ō > ū dōmē > S. ✧ PE17/152
dūme > dūme ō > ū dōmē > S. ✧ SA/dú
gōrikuba > gūrikuba ō > ū gōr(i)kubā > S. gurgof ✧ PE22/155
l̥ōko > l̥ūko ō > ū (s)lōkō > S. lhûg ✧ PE17/160
lōθo > lūθo ō > ū loth > S. lúth ✧ PE17/161
mbelekōre > mbelekūre ō > ū mbelek-óre > S. Belegûr ✧ PE17/115
ŋgōle > ŋgūle ō > ū ngōl- > S. gûl ✧ PE17/79
ŋgōle > ŋgūle ō > ū ngol- > S. gûl ✧ SA/gûl
ŋgōle > ŋgūle ō > ū NGOL > S. gûl ✧ WJ/383
onōre > onūre ō > ū NOR > S. onur ✧ PE17/168
oθθōja > oθθūja ō > ū otsōyā > S. othui ✧ VT42/25
r̥ōn > r̥ūn ō > ū SRON > srōn > S. rhû ✧ PE17/183
tindōmihelde > tindūmihelde ō > ū tindōmi-sel(dĕ) > tindúmhihel > S. Tinnúviel ✧ PE19/73
ōre > ūre ō > ū ʒorē > S. gûr ✧ VT41/11
xīθilōme > xīθilūme ō > ū χīþilōmē > hithlũṽ > S. Hithlũ ✧ PE17/133
xō > xū ō > ū > S. ✧ PE17/86

ON. [ē], [ō] became [ī], [ū]; [ē|ō] > [ī|ū]

References ✧ PE18/46; PE22/27

Variations

Order (01000)

After 00800 first in pair of voiced stops vocalized ᴹ✶negdē > N. nīdh EtyAC/NEG
Before 01100 [ɣ] vanished ᴹ✶teʒē > N. Ety/TEƷ
Before 04500 [j] vanished before [i], [ui] ᴹ✶at-yēn-ar > N. edinar Ety/YEN

Phonetic Rule Elements

[ē] > [ī] ✧ PE19/25 (ē > ī)
[ō] > [ū] ✧ PE19/25 (ō > ū)

Phonetic Rule Examples

andajēn > andajīn ē > ī ᴹ√YEN > N. ennin ✧ Ety/YEN
atjēnar > atjīnar ē > ī ᴹ✶at-yēn-ar > N. edinar ✧ Ety/YEN
dēn > dīn ē > ī ᴹ√DEN > N. dîn ✧ Ety/DEN
dēr > dīr ē > ī ᴹ√DER > ON. dîr ✧ Ety/DER
jēn > jīn ē > ī ᴹ√YEN > N. în ✧ Ety/YEN
jēnrinde > jīnrinde ē > ī ᴹ√YEN > iðrind > N. iðrin ✧ Ety/RIN
jēnrinde > jīnrinde ē > ī ᴹ✶ı̯en-rinde > N. îðrin ✧ Ety/YEN
jēre > jīre ē > ī ᴹ√YER > N. îr ✧ EtyAC/YER
kēn > kīn ē > ī ᴹ√KEN > cên > N. cîn ✧ EtyAC/KEN
kʰēril > kʰīril ē > ī ᴹ√KHER > ON. khíril ✧ Ety/KHER
lassepelēnē > lassepelīnē ē > ī ᴹ✶lassekwelēnē > N. lhasbelin ✧ EtyAC/KWEL
mēl > mīl ē > ī ᴹ√MEL > N. mîl ✧ Ety/MEL
nēdē > nīdē ē > ī ᴹ✶negdē > N. nīdh ✧ EtyAC/NEG
nēre > nīre ē > ī ᴹ✶neñrē > N. nîr ✧ EtyAC/NEI
nētʰē > nītʰē ē > ī ᴹ✶nēthē > N. nîth ✧ Ety/NETH
pantjēnare > pantjīnare ē > ī ᴹ√KWAT > N. penninar ✧ Ety/YEN
pelēnē > pelīnē ē > ī ᴹ✶kwelēnē > N. belin ✧ Ety/LAS¹
randēr > randīr ē > ī ᴹ√RAN > N. rhandir ✧ Ety/RAN
sēde > sīde ē > ī ᴹ√SED > N. sîdh ✧ Ety/SED
sparadēr > sparadīr ē > ī ᴹ√SPAR > N. feredir ✧ Ety/SPAR
tēɣ > tīɣ ē > ī ᴹ✶tēñe > N. ✧ Ety/TEÑ
tēra > tīra ē > ī ᴹ✶tēra > N. tîr ✧ Ety/TEƷ
trē > trī ē > ī ᴹ√TER > ON. trī ✧ Ety/TER
trēwā > trīwā ē > ī ᴹ✶terḗwā > N. trîw ✧ Ety/TER
tʰēa- > tʰīa- ē > ī ᴹ√THĒ > N. thio ✧ Ety/THĒ
tʰērē > tʰīrē ē > ī ᴹ✶thērē > N. thîr ✧ Ety/THĒ
xēro > xīro ē > ī ON. khēro > N. hîr ✧ Ety/KHER
arɣandōr > arɣandūr ō > ū ᴹ✶arʒā + ndōrē > arʒandōrǝ > ON. Arandur ✧ PE21/32
brōnā > brūnā ō > ū ᴹ✶b’rōnā > ON. brūna ✧ Ety/BORÓN
dō > dū ō > ū ᴹ√DOƷ/DÔ > N. ✧ Ety/DOƷ
dōlindē > dūlindē ō > ū ᴹ√DOƷ/DÔ > dúlind > N. dúlin(n) ✧ Ety/DOƷ
dōme > dūme ō > ū ᴹ✶dōmi > dūmh > N. ✧ SD/302
dōmilindē > dūmilindē ō > ū ᴹ✶dōmilindē > N. duilin ✧ SD/302
dōr > dūr ō > ū ᴹ√DOƷ/DÔ > N. dûr ✧ Ety/DOƷ
gorōmē > gorūmē ō > ū ᴹ✶Górōmē > N. Goru ✧ EtyAC/GÓROM
jōr > jūr ō > ū ᴹ√YOR > N. iûr ✧ EtyAC/YAR
kʰīsislōmē > kʰīsislūmē ō > ū ᴹ✶χīsi-slōmē > N. Cílu ✧ PE21/32
kʰōgore > kʰūgore ō > ū ᴹ✶Khōgore > N. Huor ✧ Ety/GOR
kʰōn > kʰūn ō > ū ᴹ√KHŌ-N > N. hûn ✧ Ety/KHŌ-N
kʰōpa > kʰūpa ō > ū ᴹ√KHOP > N. hûb ✧ Ety/KHOP
kʰōre > kʰūre ō > ū ᴹ√KHOR > N. hûr ✧ Ety/KHOR
kʰōtʰ > kʰūtʰ ō > ū ᴹ√KHOTH > N. hûd ✧ Ety/KHOTH
lōke > lūke ō > ū ᴹ√LOK > N. lhûg ✧ Ety/LOK
lōre > lūre ō > ū ᴹ√LOR > N. lhûr ✧ EtyAC/LOS
lōsien > lūsien ō > ū ᴹ√LOS > N. Lhuien ✧ Ety/LOS
mōl > mūl ō > ū ᴹ✶mōl- > N. mûl ✧ Ety/MŌ
mōta- > mūta- ō > ū ᴹ√ > N. mudo ✧ Ety/MŌ
ŋgōle > ŋgūle ō > ū ᴹ√ÑGOL > N. gûl ✧ Ety/ÑGOL
nōre > nūre ō > ū ᴹ√ > N. nûr ✧ Ety/NŌ
nōre > nūre ō > ū ᴹ√NŌ/ONO > ON. #nūre ✧ Ety/THEL
nōro > nūro ō > ū ᴹ√ > ON. #nūro ✧ Ety/NŌ
rōma > rūma ō > ū ᴹ√ROM > ON. rūma ✧ Ety/ROM
rōmena > rūmena ō > ū ᴹ√ > N. †rhufen ✧ Ety/RŌ
rōna > rūna ō > ū ON. rōna > N. rhûn ✧ Ety/RŌ
tindōme > tindūme ō > ū ᴹ√DOM > tindumh > tindu > N. tinnu ✧ Ety/TIN
tindōmiselde > tindūmiselde ō > ū ᴹ✶Tindōmiselde > ON. tindūmhiell ✧ Ety/TIN
ōle > ūle ō > ū ᴹ✶ñōle > N. ûl ✧ Ety/ÑOL
ōne > ūne ō > ū ᴹ✶ñōn- > N. ûn ✧ EtyAC/ÑŌ¹
ōno > ūno ō > ū ᴹ√ÑŌ²/ÑONO > N. ûn ✧ EtyAC/ÑŌ²
ōno > ūno ō > ū ᴹ√ONO > N. ûn ✧ Ety/ONO
wanōrō > wanūrō ō > ū ᴹ✶wa-nōrō > ON. wanūro ✧ Ety/TOR

G. [ē], [ō] became [ī], [ū]; [ē|ō] > [ī|ū]

HPG/§1.1

Order (01100)

Before 04900 [ll] became [l] lengthening preceding stressed vowel ᴱ✶oldō > G. ôla GL/62

Phonetic Rule Elements

[ē] > [ī] ✧ GG/14 (ē > ī); PE15/13 (ê > î; ê + r)
[ō] > [ū] ✧ GG/14 (ō > ū); PE15/13 (ô > û; ô + r)

Phonetic Rule Examples

gʷē > gʷī ē > ī ᴱ√u̯ē· > G. Gwî ✧ GL/45
gʷērī > gʷīrī ē > ī ᴱ√gu̯er > G. gwîri ✧ GL/46
gʷēs > gʷīs ē > ī #ᴱ√gwes- > G. gwîs ✧ GL/47
nēr > nīr ē > ī ᴱ✶nı̯ēr- > G. nîr¹ ✧ GL/60
tē > tī ē > ī ᴱ√ > G. ✧ GL/69
xēl > xīl ē > ī ᴱ√χele-k > G. hîl ✧ GL/49
gʷōri > gʷūri ō > ū ᴱ√GWORI > G. gur ✧ QL/104
ndōl > ndūl ō > ū ᴱ√ndolo- > G. dûl ✧ GL/30