The Primitive Elvish long vowel [ǭ], a slacker and lower long [ō], developed into [au] in Sindarin (PE18/96). A similar sound change occurred in Noldorin, though the exact details of the phonetic development are different. Tolkien described this sound change in several places, with somewhat different details each time:

In the earliest recorded Noldorin, [ē, ǣ, ā, ǭ, ō] remained distinct though changed to [ī, ei, ǭ, ou, ū]; later ei, ou products of ǣ, ǭ became ai, au coalescing with original ai, au (first version of Tengwesta Qenderinwa [TQ1], 1930s, PE18/46).

   é called (amba-wai) “raised-w”. This was frequently used above letters to denote a following w: thus dé = khw, 7é = rw, etc. Its most regular use was to express the diphthongs au̯, and the archaic ou̯ from PQ ǭ which later became au̯: thus ]é [au̯] (and archaic hé [ou̯]). These were seldom written ]., or h. (Feanorian Alphabet, 1930s, PE22/27).

In the earliest Noldorin ē, ę̄, ā, ǭ, ō appear as ī, ę̄, ǭ, ǭ, ū; later ę̄, ǭ coalesce with as ai, au (second version of Tengwesta Qenderinwa [TQ2], circa 1950 written shortly before the change of Noldorin to Sindarin, PE18/96).

The sound [ǭ] was fairly common in Old Sindarin and Old Noldorin, since primitive [ā] became [ǭ], and long [ā] was very common in Primitive Elvish. Most examples of this phonetic development are of this origin: [ā] > [ǭ] > [au]. However, this sound also arose as the a-fortification of o in Primitive Elvish. Some of the cleanest examples of the sound change come from this original primitive [ǭ], since there are fewer intermediate changes to examine. Here are some clear examples of this phonetic development:

• Kh. Khazād > chaðǭd > chaðaud > S. Hadhod “Dwarf” (PE17/153).
• ✶Orǭmē > Oraúmh > Araúv > S. Áraw “Orome” (PE17/153).
• ᴹ✶rǭda > ON. rauda > N. rhaudh “hollow, cavernous” (Ety/ROD; EtyAC/ROD).
• ᴹ✶ndākō > ON. ndōko > N. daug “warrior, soldier” (Ety/NDAK).

The last example is somewhat obscure because Tolkien frequently represented [ǭ] as ō in The Etymologies; the long vowel [ō] could not itself appear in Old Noldorin, since earlier long [ō] became [ū]. It is possible, though, that in some conditions [ǭ] became [ō] instead: see the entry on how [au] became [o] in polysyllables. This later development of diphthongal [au] to short [o] frequently masked the earlier change of [ǭ] > [au], as shown in the final syllables of the first example Hadhod above.

In the Comparitive Tables for phonetic developments composed in the 1930s, Tolkien shows a more intricate development than the one given above. In particular, all of ā, ǭ and au merged into ǭ, which then becomes ou and then finally au (PE19/25): ā/ǭ/au > ǭ > ou > au. The same intermediate development ǭ > ou > au appears in the Noldorin notes from the 1930s on how ǭ evolved (PE18/46, PE22/27); see above for details. There are examples showing this fuller development in The Etymologies:

• ᴹ✶Orǭmē > Oroume > ON. Araume > N. Araw “Orome” (Ety/ORÓM).
• ᴹ✶Phay-anāro > ON. Phayanṓr > Feanoúr > N. Féanor (Ety/PHAY).

The second example does not show the intermediate form *Feanaur before the [au] became [o] in the final syllable, but is otherwise consistent with the same ǭ > ou > au development as the first example. There is no evidence that Sindarin shared this more complex development, but there is a note from Tolkien stating that the later diphthong [ou] became [au] (PE17/99). This sound change could not have applied to the primitive diphthong [ou], which became [ū] in Sindarin:

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

Thus, it is conceivable the more complex series of sound changes ǭ > ou > au applied to Sindarin as well, and we simply have no examples of it.

Conceptual Development: There are no clear indications of ǭ in Tolkien’s earliest conceptions of Primitive Elvish from the 1910s and 1920s. It was introduced into Primitive Elvish in the 1930s, and thereafter the Noldorin and Sindarin developments seem to be mostly the same, except for the intermediate Noldorin sound change to ou (which may or may not have applied in Sindarin).

GS/§4.109 @@@ Orǭmē > Oroume > ON. Araume implies sound change occurred before final vowel loss, but plurals like naug (< nǭka < nauka) vs. nuig (< nǭik < nǭki) imply it occurred after final vowel loss)