S. [iu] and [ju] became [ȳ]; [{ij}u|jui] > [ȳ|jui]
In Sindarin, the diphthong [iu] developed into [ȳ], a change mentioned in The Lord of the Rings Appendix E (LotR/1115), and also in a chart describing the phonetic development of diphthongs appearing in notes associated with Tolkien’s discussion of Hands, Fingers and Numerals from the late 1960s (VT48/7). In both cases, it seems this phonetic development was the result of both original primitive [iu] as well as the diphthongs that developed when primitive [eu] became [iu]. This sound change also occurred from initial yu- [ju-], as noted by David Salo (GS/§4.92, §4.93). He suggested [ju] > [jy] > [ȳ], but I think [ju] > [iu] > [ȳ] is also possible:
The change of [ju] > [ȳ] might have been inhibited where the u was already part of a diphthong, as in N. iuith [juiθ] “use”, as was also suggested by David Salo (GS/§4.93). Since the Noldorin phonetic developments of these diphthongs were different than Sindarin, it’s hard to say for sure.
Conceptual Development: Other than a general merging of primitive [eu] and [iu], these phonetic developments were quite different at earlier conceptual stages of the language. See the entry on how [eu] became [iu] for further details, along with more examples of this sound change.
Reference ✧ LotR/1115 ✧ for example: y < ✶eu, y < ✶iu
Order (01200)
Before | 05100 | [au], [ae] became [o], [e] in polysyllables | ✶keu̯rānă > cýrawn > S. cýron | VT48/7 |
Phonetic Rule Elements
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✧ LotR/1115 (eu > y); LotR/1115 (iu > y); VT48/7 (iu > ȳ) |
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Phonetic Rule Examples
diule > dȳle | iu > ȳ | ✶deulē > S. dýl | ✧ PE17/151 |
diura > dȳra | iu > ȳ | √DEWE > S. dýr | ✧ PE17/151 |
kiule > kȳle | iu > ȳ | ✶keule > S. cŷl | ✧ VT48/8 |
kiura- > kȳra- | iu > ȳ | √kewe > S. cýra | ✧ VT48/7 |
kiura > kȳra | iu > ȳ | ✶keu-rā > S. cŷr | ✧ VT48/8 |
kiurǭna > kȳrǭna | iu > ȳ | ✶keu̯rānă > cýrawn > S. cýron | ✧ VT48/7 |
liuka > lȳka | iu > ȳ | √LEWEK > S. lýg | ✧ PE17/160 |
iu > ȳ | iu > ȳ | ✶eu > S. y | ✧ LotR/1115 |
julma > ȳlma | ju > ȳ | ✶julmā > S. ylf | ✧ WJ/416 |
jūneke > ȳneke | ju > ȳ | ✶yūneke > S. ýneg | ✧ VT47/41 |