In both Sindarin and Noldorin, at a late stage of the language, the diphthong [ou] became [au], as noted by David Salo (GS/§4.110). This development is more straightforward in Noldorin, where primitive [ou] also ultimately developed into [au]:

• ᴹ√NOWO > N. naw “idea” (Ety/NOWO).
• ᴹ√TOW > N. taw “of wool, woollen” (Ety/TOW).

There are three other primitive vowel combinations that produced [au] in Noldorin: [ā], [ǭ] and [au]. According to the Comparative Tables of phonetic development from the 1930s, all three sounds passed through [ou] on their way to becoming [au]: ā, ǭ, au > ǭ > ǫu > au (PE22/25). There are a number of examples in The Etymologies that demonstrate these phonetic developments:

• ᴹ✶Phayanāro > ON. Phayanṓr > Feanoúr [> *Feanaur] > N. Feanor “Radiant Sun” (Ety/PHAY).
• ᴹ✶Orǭmē > Oroume > ON. Araume > N. Araw “Orome” (Ety/ORÓM).
• ᴹ√DOƷ > dogme > dongme > ON. doume > N. daw “night-time, gloom” (Ety/DOƷ).

Note that the Comparative Tables do not show primitive [ou] > [au] in Noldorin, but instead show [ou] > [ū] (PE22/25). Although this phonetic development is not consistent with the Noldorin examples in The Etymologies, by the Sindarin of the 1950s and later, all primitive [ou] became [ū] (VT48/7). Furthermore, while primitive [ā], [ǭ], [au] all ultimately became [au] in Sindarin, there is no sign that they passed through [ou] in this process.

Nevertheless, there is one statement by Tolkien indicating that, where the diphthong [ou] arose at a later stage of phonetic development in Sindarin, it developed into [au]:

Q Orome gives S Araw, from Oromē. > Orom̆ > Orow̯ (so finally) > Araw. ow new > aw, but old ow- > ū (PE17/99).

In this example, a “new” [ou] became [au] whereas an “old” (primitive) [ou] became [ū], as noted above. This specific example is a bit ambiguous, because elsewhere Tolkien typically gave ✶Orǭmē or Arǭmēz as the primitive form of Araw (PE17/138, 153; WJ/400; Ety/ORÓM; PE22/037), where [ǭ] > [au] as usual. Nevertheless, it seems likely that in those rare circumstances where [ou] arose as a late diphthong in Sindarin, it developed into [au].

Conceptual Developments: There are no indications that the diphthong [ou] ever became [au] in the Gnomish of the 1910s. There is an Early Noldorin diphthongal development chart from the 1920s that shows [ou] > [au] (PE15/64), but the few Early Noldorin examples of this sound change seem to show [ou] > [ū] instead. For further discussion on how the phonetic development of [ou] changed in the various conceptual stages of the languages, see the entry on how [ei], [ou] became [ī], [ū] in Old Sindarin.

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